Chapter

Evapotranspiration and Evaporative Demand

Authors:
  • USDA-ARS Southeast Area, Stoneville, MS 38776
To read the full-text of this research, you can request a copy directly from the authors.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

Article
Full-text available
Increased atmospheric evaporative demand has important implications for humans and ecosystems in water-scarce lands. While temperature plays a significant role in driving evaporative demand and its trend, other climate variables are also influential and their contributions to recent trends in evaporative demand are unknown. We address this gap with an assessment of recent (1980–2020) trends in annual reference evapotranspiration (ETo) and its drivers across the continental United States based on five gridded datasets. In doing so, we characterize the structural uncertainty of ETo trends and decompose the relative influences of temperature, wind speed, solar radiation, and humidity. Results highlight large and robust changes in ETo across much of the western United States, centered on the Rio Grande region where ETo increased 135–235 mm during 1980–2020. The largest uncertainties in ETo trends are in the central and eastern United States and surrounding the Upper Colorado River. Trend decomposition highlights the strong and widespread influence of temperature, which contributes to 57% of observed ETo trends, on average. ETo increases are mitigated by increases in specific humidity in non-water-limited regions, while small decreases in specific humidity and increases in wind speed and solar radiation magnify ETo increases across the West. Our results show increases in ETo across the West that are already emerging outside the range of variability observed 20–40 years ago. Our results suggest that twenty-first-century land and water managers need to plan for an already increasing influence of evaporative demand on water availability and wildfire risks. Significance Statement Increased atmospheric thirst due to climate warming has the potential to decrease water availability and increase wildfire risks in water-scarce regions. Here, we identified how much atmospheric thirst has changed across the continental United States over the past 40 years, what climate variables are driving the change, and how consistent these changes are among five data sources. We found that atmospheric thirst is consistently emerging outside the range experienced in the late twentieth century in some western regions with 57% of the change driven by temperature. Importantly, we demonstrate that increased atmospheric thirst has already become a persistent forcing of western landscapes and water supplies toward drought and will be an essential consideration for land and water management planning going forward.
Thesis
Full-text available
Long-term observations of pan evaporation and water budget-derived evapotranspiration across the conterminous United States provide the first observational evidence supporting the hypothesis of a complementary relationship in regional evapotranspiration, in terms both of the evaporation rates themselves and of long-term trends in their component dynamics. The conjectured relationship has now become an observational fact. To establish a baseline for the study of climate change and/or variability, a complementary relationship model estimates spatially distributed actual evapotranspiration across the conterminous US on a monthly basis over a recent 42-year period. This is used to examine two advective calibration approaches, and trends in actual evapotranspiration and its components as to their direction, magnitude, statistical significance, and spatial distributions. In observations of trends in ETa and in its two component trends—the radiative energy and regional advective dynamics—it is shown that, contrary to previous conclusions that have been predicated on questionable and restrictive assumptions over near-continental scales, trends in the components must be examined concurrently within the context of the complementary relationship to explain trends in regional ETa. It is further shown that only by examining spatially concurrent observations at smaller spatial scales can good conclusions be drawn about the “strength” of the complementary relationship, specifically, and ET trends in general. The two most problematic data sets used in the analysis are improved: solar radiation, which suffers from the effects of local topography; and pan evaporation, which bears the effects of anthropogenic heterogeneities inherent in a variously sourced data set. A procedure is developed to mitigate the confounding influence of rugged terrain on the analysis of the short-wave radiative balance, producing a long-term, high-resolution, topographically corrected net radiation data set. Twelve years of missing diffuse radiation data are replicated based on their historical relationships to coincident, contemporaneous direct normal and global radiation. A monthly topographic correction factor is derived to account for the incidence of direct solar radiation on arbitrarily oriented ground surfaces at any latitude throughout the diurnal and annual cycles. The factor is applied to spatially interpolated surfaces of monthly direct solar radiation which, when added to surfaces of diffuse radiation, provide the total incident solar radiation input to an existing energy budget. This yields the net surface radiation that may then applied in evapotranspiration models. Pan evaporation observations are gathered from two data sources for 228 pans across the conterminous US toward an examination of long-term trends in annual and warmseason totals. The data are characterized by their incompleteness and, more importantly, non-homogeneity that, unless accounted for, can introduce spurious biases into analyses of long-term trends. However, what scant metadata are available are elliptical. The need to retain climatically driven trends after homogenization requires a technique that resolves understandings of both the physical dynamics and the statistical properties of the data by combining objective rigor with subjective judgment. Using the t-test to indicate statistically significant abrupt shifts in each pan’s time-series, 172 pans are adjusted at a total of 326 abrupt data-shifts, adjusting 43% of the annual data and 55% of the warmseason data. Comparing trend results from pre- and post-adjustment data across all pans, some differences are noted in the details, but they are not together significant enough to change the conclusions of the trend analysis. Pan evaporation has decreased at 64% of the year-round pans in the conterminous US over the past half-century. The so-called “Pan Evaporation Paradox” is shown to be no more than a manifestation of the natural complementarity between actual and potential evapotranspiration. An examination of trends in the radiative energy and regional advective components of basin-derived actual evapotranspiration shows that both components must be considered together to explain the relationship between actual and potential evapotranspiration. Actual evapotranspiration is modeled using a regional, seasonal Advection-Aridity approach to create a spatially distributed, monthly time-series for a 42-year period at a 5-km resolution over the conterminous US. Formulations of both dynamics in the evaporative process are improved with respect to the applicability of the model across large topographic and climatic variations. The radiative input is the aforementioned topographically corrected data set. The advective input is improved by analysis of two regional calibrations of the wind function: first such that modeled actual evapotranspiration matches basin-derived evapotranspiration at 655 basins across the conterminous US; second such that potential evapotranspiration matches point observations of pan evaporation across the southern tier of states. Each calibration invokes different assumptions and limitations on its applicability in the temporal and spatial domains. The parameter sets of the derived wind functions are similar in value, but the first is noisier while the second bears less significant functional relationships to wind speed. The modeled annual evapotranspiration data are verified against observed water budget-derived actual evapotranspiration. The basin-derived calibration of the wind function performs the best, while the pan evaporation-based calibration under-estimates evapotranspiration. In purely statistical terms, the basin-derived calibration is preferred, but the performances of both calibrations bear functional relationships to the precipitation, basin-derived evapotranspiration, and wind speed in the areas of application. In terms of long-term trends over the modeling period WY 1953-1994, for the conterminous US as a whole, a 4.2% decrease in modeled annual actual evapotranspiration is observed for the basin-derived calibration, a trend significant at the 69% confidence level according to the Mann-Kendall test. Over the southern tier of states, a 3.1% decrease in modeled annual actual evapotranspiration is observed for the basin-derived calibration (significant at 62%), and a 2.1% decrease in modeled annual actual evapotranspiration for the pan evaporation-derived calibration, (significant at 47%). Reducing the spatial scale of trend-analyses—from the continental US through the nested component 18 Water Resource Regions and the further-nested 334 Accounting Units to the 655 relatively undisturbed basins across the continental US—allows for clearer identification of areas with significant trends, and the breakdown into component dynamics shows that trends in actual evapotranspiration can be determined to originate in either energy or water fluxes, or both.
Article
Full-text available
Evapotranspiration is one of the major components of the water balance and has been identified as a key factor in hydrological modelling. For this reason, several methods have been developed to calculate the reference evapotranspiration (ET<sub>0</sub>). In modelling reference evapotranspiration it is inevitable that both model and data input will present some uncertainty. Whatever model is used, the errors in the input will propagate to the output of the calculated ET<sub>0</sub>. Neglecting information about estimation uncertainty, however, may lead to improper decision-making and water resources management. One geostatistical approach to spatial analysis is stochastic simulation, which draws alternative and equally probable, realizations of a regionalized variable. Differences between the realizations provide a measure of spatial uncertainty and allow to carry out an error propagation analysis. Among the evapotranspiration models, the Hargreaves-Samani model was used. The aim of this paper was to assess spatial uncertainty of a monthly reference evapotranspiration model resulting from the uncertainties in the input attributes (mainly temperature) at regional scale. A case study was presented for Calabria region (southern Italy). Temperature data were jointly simulated by conditional turning bands simulation with elevation as external drift and 500 realizations were generated. The ET<sub>0</sub> was then estimated for each set of the 500 realizations of the input variables, and the ensemble of the model outputs was used to infer the reference evapotranspiration probability distribution function. This approach allowed to delineate the areas characterized by greater uncertainty, to improve supplementary sampling strategies and ET<sub>0</sub> value predictions.
Article
Full-text available
To fully attribute the variability of reference evapotranspiration to its drivers, a mean-value, second-moment uncertainty analysis is applied to a 30-year, CONUS-wide reanalysis of daily and annual tall-crop reference evapotranspiration as estimated by the ASCE Standardized Reference Evapotranspiration Equation driven by four variables from the North American Land Data Assimilation System phase 2 (NLDAS-2) reanalysis: temperature, specific humidity, wind speed, and downward shortwave radiation. The attribution methodology accounts for both the sensitivity of reference evapotranspiration to its drivers and their observed variabilities, and it permits the decomposition of reference evapotranspiration variability across CONUS at various timescales into contributions from each driver. An analytically derived expression of the sensitivity of daily ASCE Standardized Reference ET to each of the drivers is provided and mapped. Contrary to the assumption of much hydrologic practice, temperature is neither always nor everywhere the most significant driver of temporal variability in reference evapotranspiration. Instead, depending on regional hydroclimatology, season, and timescale, different drivers dominate; in fact, in many regions, temperature-based parameterizations should be avoided at all timescales. © 2016 American Society of Agricultural and Biological Engineers.
Article
Full-text available
Changes in the global water cycle can cause major environmental and socioeconomic impacts. As the average global temperature increases, it is generally expected that the air will become drier and that evaporation from terrestrial water bodies will increase. Paradoxically, terrestrial observations over the past 50 years show the reverse. Here, we show that the decrease in evaporation is consistent with what one would expect from the observed large and widespread decreases in sunlight resulting from increasing cloud coverage and aerosol concentration.
Article
Full-text available
To understand the sources of temporal and spatial variability of atmospheric evaporative demand across the conterminous United States (CONUS), a mean-value, second-moment uncertainty analysis is applied to a spatially distributed dataset of daily synthetic pan evaporation for 1980-2009. This evaporative demand measure is from the "PenPan" model, which is a combination equation calibrated to mimic observations from U.S. class-A evaporation pans and here driven by six North American Land Data Assimilation System variables: temperature, specific humidity, station pressure, wind speed, and downwelling shortwave and longwave radiation. The variability of evaporative demand is decomposed across various time scales into contributions from these drivers. Contrary to popular expectation and much hydrologic practice, temperature is not always the most significant driver of temporal variability in evaporative demand, particularly at subannual time scales. Instead, depending on the season, one of four drivers (temperature, specific humidity, downwelling shortwave radiation, and wind speed) dominates across different regions of CONUS. Temperature generally dominates in the northern continental interior. This analysis assists land surface modelers in balancing parameter parsimony and physical representativeness. Patterns of dominant drivers are shown to cycle seasonally, with clear implications for modeling evaporative demand in operational hydrology or as a metric of climate change and variability. Depending on the region and season, temperature, specific humidity, downwelling shortwave radiation, and wind speed must together be examined, with downwelling longwave radiation as a secondary input. If any variable may be ignored, it is atmospheric pressure. Parameterizations of evaporative demand based solely on temperature should be avoided at all time scales.
Article
Full-text available
Due to the influence of evaporation on land-surface temperature, thermal remote sensing data provide valuable information regarding the surface moisture status. The Atmosphere-Land Exchange Inverse (ALEXI) model uses the morning surface temperature rise, as measured from a geostationary satellite platform, to deduce surface energy and water fluxes at 5-10 km resolution over the continental United States. Recent improvements to the ALEXI model are described. Like most thermal remote sensing models, ALEXI is constrained to work under clear-sky conditions when the surface is visible to the satellite sensor, often leaving large gaps in the model output record. An algorithm for estimating fluxes during cloudy intervals is presented, defining a moisture stress function relating the fraction of potential evapotranspiration obtained from the model on clear days to estimates of the available water fraction in the soil surface layer and root zone. On cloudy days, this stress function is inverted to predict the soil and canopy fluxes. The method is evaluated using flux measurements representative at the watershed scale acquired in central Iowa with a dense flux tower network during the Soil Moisture Experiment of 2002 (SMEX02). The gap-filling algorithm reproduces observed fluxes with reasonable accuracy, yielding ∼20% errors in ET at the hourly timescale, and 15% errors at daily timesteps. In addition, modeled soil moisture shows reasonable response to major precipitation events. This algorithm is generic enough that it can easily be applied to other thermal energy balance models. With gap-filling, the ALEXI model can estimate hourly surface fluxes at every grid cell in the U.S. modeling domain in near real-time. A companion paper presents a climatological evaluation of ALEXI-derived evapotranspiration and moisture stress fields for the years 2002-2004.
Article
Full-text available
An update is provided on the Earth's global annual mean energy budget in the light of new observations and analyses. In 1997, Kiehl and Trenberth provided a review of past estimates and performed a number of radiative computations to better establish the role of clouds and various greenhouse gases in the overall radiative energy flows, with top-of-atmosphere(TOA) values constrained by Earth Radiation Budget Experiment values from 1985 to 1989, when the TOA values were approximately in balance. The Clouds and the Earth's Radiant Energy System (CERES) measurements from March 2000 to May 2004 are used at TOA but adjusted to an estimated imbalance from the enhanced greenhouse effect of 0.9 W m(-2). Revised estimates of surface turbulent fluxes are made based on various sources. The partitioning of solar radiation in the atmosphere is based in part on the International Satellite Cloud Climatology Project (ISCCP) FD computations that utilize the global ISCCP cloud data every 3 h, and also accounts for increased atmospheric absorption by water vapor and aerosols. Surface upward longwave radiation is adjusted to account for spatial and temporal variability. A lack of closure in the energy balance at the surface is accommodated by making modest changes to surface fluxes, with the downward longwave radiation as the main residual to ensure a balance. Values are also presented for the land and ocean domains that include a net transport of energy from ocean to land of 2.2 petawatts (PW) of which 3.2 PW is from moisture (latent energy) transport, while net dry static energy transport is from land to ocean. Evaluations of atmospheric re-analyses reveal substantial biases.
Article
Full-text available
This study emerged from the declines in pan evaporation reported across many regions worldwide (Roderick et al., 2009a; Roderick et al., 2009b). Our aim is to fully investigate this phenomenon. We constructed an instrumented Class A pan at the Bureau of Meteorology field station at Canberra Airport in 2007 to study pan evaporation under non-steady state conditions. The pan has instruments that allow us to measure evaporation rates over 5 minute intervals as well as incoming- and outgoing-radiation (shortwave and longwave), skin water temperature, temperature in- and out-side the pan at different depths (25mm, 150mm, 175mm) with wind speed and humidity measured at 2 m above the ground level. The instrumented pan is designed to replicate an operational Class A pan installation. Here we describe the instrumentation and present preliminary analysis of vapour transfer from pan water surface. References: Roderick, M.L., Hobbins, M.T. and Farquhar, G.D., 2009a. Pan Evaporation Trends and the Terrestrial Water Balance. I. Principles and Observations. Geography Compass, 3(2): 746-760. Roderick, M.L., Hobbins, M.T. and Farquhar, G.D., 2009b. Pan Evaporation Trends and the Terrestrial Water Balance. II. Energy Balance and Interpretation. Geography Compass, 3(2): 761-780.
Article
Full-text available
From the 1950s to the 1980s, a significant decrease of surface solar radiation has been observed at different locations throughout the world. Here we show that this phenomenon, widely termed global dimming, is dominated by the large urban sites. The global-scale analysis of year-to-year variations of solar radiation fluxes shows a decline of 0.41 W/m2/yr for highly populated sites compared to only 0.16 W/m2/yr for sparsely populated sites (<0.1 million). Since most of the globe has sparse population, this suggests that solar dimming is of local or regional nature. The dimming is sharpest for the sites at 10°N to 40°N with great industrial activity. In the equatorial regions even the opposite trend to dimming is observed for sparsely populated sites.
Article
Full-text available
The main objective of this study is to investigate the effects of climatic parameters variability on evapotranspiration in five climatologically different regions of Iran. The regions include Tehran, Esfahan, Shiraz, Tabriz and Mashhad. Fifty four-year monthly records of temperature, relative humidity, sunshine duration, wind speed, and precipitation depth from 1951 to 2005 comprise the database. Trend and persistence analyses of the data are performed using the Mann–Kendall test, the Cumulative Deviation test, Linear Regression, and the Autocorrelation Coefficient. A sensitivity analysis of meteorological variables in these five regions is carried out using Penman-Monteith formula. In all of the studied regions, sensitivity analysis reveals that, temperature and relative humidity are the most sensitive parameters in Penman-Monteith formula respectively. The results of this study indicate that the effective climatic variables on evapotranspiration are changing, though in each region the variables have significant long-term trends and persistence.
Article
Full-text available
Evaporative demand, measured by pan evaporation, has declined in many regions over the last several decades. It is important to understand why. Here we use a generic physical model based on mass and energy balances to attribute pan evaporation changes to changes in radiation, temperature, humidity and wind speed. We tested the approach at 41 Australian sites for the period 1975–2004. Changes in temperature and humidity regimes were generally too small to impact pan evaporation rates. The observed decreases in pan evaporation were mostly due to decreasing wind speed with some regional contributions from decreasing solar irradiance. Decreasing wind speeds of similar magnitude has been reported in the United States, China, the Tibetan Plateau and elsewhere. The pan evaporation record is invaluable in unraveling the aerodynamic and radiative drivers of the hydrologic cycle, and the attribution approach described here can be used for that purpose.
Article
Full-text available
By combining the complementary relationship of evaporation with the coupled long-term water-energy balance of Porporato et al. (2004) in a Budyko-type framework, one can, from atmospheric measurements alone, derive important ecosystem characteristics, such as the mean effective relative soil moisture and the maximum soil water storage, as well as predict changes in the rooting depth of vegetation as a response to climate variations.
Conference Paper
Crop coefficients derived from properly designed, operated and maintained lysimeters provide the most accurate values throughout the growing season and are critical in the computation of hourly and daily, regionally based, crop evapotranspiration (ET) values. Multi-stage crop coefficients can be derived from continuously recording lysimeters, increasing the accuracy of both daily and seasonal irrigation crop demand estimates. These crop coefficients can be used with calculated, network based, reference crop ET to develop and disseminate locally representative crop water use estimates. Subsequently, using these accurate values in estimating crop water demand results in improved validity of regional water demand models, better assessments of proposed water policy measures, and enhanced integrity with crop consultants, water districts, and agricultural producers, ultimately resulting in better (more efficient) irrigation management and water conservation.
Article
Pan evaporation (ETpan) has decreased at 64% of pans in the conterminous U.S. over the past half-century. Comparing trends in ETpan and water budget-derived actual evapotranspiration (ET*a), we observe the so-called “Pan Evaporation Paradox,” which we confirm is no more than a manifestation of the complementarity between actual evapotranspiration (ETa) and potential evapotranspiration (ETp). Examining trends in the components of ETa—the radiative energy and regional advective budgets—we show that both components must be considered together to explain the relationship between ETpan and ET*a.
Article
[1] Using independent observations of actual and potential evapotranspiration at a wide range of spatial scales, we provide direct observational evidence of the complementary relationship in regional evapotranspiration hypothesized by Bouchet in 1963. Bouchet proposed that, for large homogeneous surfaces with minimal advection of heat and moisture, potential and actual evapotranspiration depend on each other in a complementary manner through land-atmosphere feedbacks. Although much work has been done that has led to important theoretical and conceptual insights about regional actual evapotranspiration and its relation to regional potential evapotranspiration, never before has a data set of direct observations been assembled that so clearly displays complementarity, providing strong evidence for the complementary relationship hypothesis, and raising its status above that of a mere conjecture.
Article
IPCC (2007): Climate Change The Physical Science Basis The Intergovernmental Panel on Climate Change (IPCC) was jointly established by the World Meteorological Organization and the United Nations Environment Programme (UNEP) in 1988. The purpose of IPCC is to assess available information on the science of climate change and to provide policy-relevant but not policy- prescriptive assessments of interest to policymakers, scientists, and the public. IPCC released its fourth Working Group 1 (WG1) assessment report on the state of understanding of the physical science basis of climate change in Paris in February 2007. This talk will summarize the key scientific findings of that report, including observations of changes in the atmosphere, ocean, and ice, and in forcing agents such as carbon dioxide and aerosol, advances in understanding of attribution of changes in climate, and projections of future changes in coming decades and beyond. Finally, a brief summary of the author's view of future challenges for research and for IPCC will be presented.
Article
The observed reduction in land surface radiation over the last several decades (1960-1990), the so-called ``dimming effect,'' and the more recent evidence of a reversal in ``dimming'' over some locations beyond 1990 suggest several consequences on climate, notably on the hydrological cycle. Such a reduction in radiation should imply reduced surface temperature (Ts) and precipitation, which have not occurred. We have investigated the possible causes for the above climate features using a climate model coupled to a dynamic ocean model under natural and anthropogenic conditions. To isolate the aerosol influence on surface radiation trends, we have analyzed transient climate simulations from 1960 to 2002 with and without anthropogenic aerosols. Based on a linear trend with aerosol effects included, the global mean change in the surface solar radiation absorbed over land is -0.021 +/- 0.0033 Wm-2yr-1. Although the overall trend is negative, we do note a reversal in dimming after 1990, consistent with observations. Without aerosol effects, the surface solar radiation absorbed over land increases throughout 1960 to 2002, mainly due to the decrease in cloud cover associated with increased greenhouse warming. In spite of a simulated increase in Ts of 0.012 K yr-1 for 1960 to 2002, the global mean latent heat flux and associated intensity of the hydrological cycle decrease overall, however with increases over some land locations due mainly to moisture advection. Simulated changes correspond more closely to observed changes when accounting for aerosol effects on climate.
Article
A monthly dataset of Palmer Drought Severity Index (PDSI) from 1870 to 2002 is derived using historical precipitation and temperature data for global land areas on a 2.58 grid. Over Illinois, Mongolia, and parts of China and the former Soviet Union, where soil moisture data are available, the PDSI is significantly correlated (r 5 0.5 to 0.7) with observed soil moisture content within the top 1-m depth during warm-season months. The strongest correlation is in late summer and autumn, and the weakest correlation is in spring, when snowmelt plays an important role. Basin-averaged annual PDSI covary closely (r 5 0.6 to 0.8) with streamflow for seven of world's largest rivers and several smaller rivers examined. The results suggest that the PDSI is a good proxy of both surface moisture conditions and streamflow. An empirical orthogonal function (EOF) analysis of the PDSI reveals a fairly linear trend resulting from trends in precipitation and surface temperature and an El Nino- Southern Oscillation (ENSO)-induced mode of mostly interannual variations as the two leading patterns. The global very dry areas, defined as PDSI ,2 3.0, have more than doubled since the 1970s, with a large jump in the early 1980s due to an ENSO-induced precipitation decrease and a subsequent expansion primarily due to surface warming, while global very wet areas (PDSI .1 3.0) declined slightly during the 1980s. Together, the global land areas in either very dry or very wet conditions have increased from ;20% to 38% since 1972, with surface warming as the primary cause after the mid-1980s. These results provide observational evidence for the increasing risk of droughts as anthropogenic global warming progresses and produces both increased temperatures and increased drying.
Article
To quantify effects of land-use history on soil spatial heterogeneity, we sampled surficial mineral soils (0–7.5 cm depth) using a spatially-explicit design within three 0.09-ha plots in each of three ecosystems in the Southern Piedmont area of USA. The three ecosystems were old hardwood forests, cultivated agricultural fields, and old-field pine forests, three common ecosystems in the region that represent the common trajectory of nearly two centuries of land-use history. In total, 243 soil samples were collected and 12 soil properties were measured on each sample. Results indicated that: (i) land-use history altered soil properties' central tendencies and their spatial heterogeneities; (ii) within-plot variations indexed by coefficients of variation and Cochran's C tests of within-plot variances were generally much higher in hardwood and pine-forest soils than in cultivated soils; (iii) for soil C, and major and trace elements spatial patterns as indicated by trend-surface analysis, correlograms, and interpolation maps were evident under hardwood and pine forests and much less so under cultivation. We document cases in which land use alters both the soil property's central tendencies and their heterogeneity (C, N, C:N ratio, Ca, and K), and cases in which changes are apparent in central tendency but not in heterogeneity (bulk density, Db). Relative to soils that have never been cultivated, spatial heterogeneity is greatly reduced in many soil properties by plowing, fertilization, and other practices associated with agricultural crop production, but that successional forest growth on previously cultivated soils re-establishes heterogeneity of soil properties within a few decades. Overall, within-plot variances were very high for most properties especially of the forested soils and indicate that researchers should better match sample sizes and sample designs with the variability of the soil properties they are studying.
Article
Surface radiation budget data are presented for the midseasonal months of July and October of 1983 and January and April of 1984. These data allow the examination of geographical and seasonal variations of the entire surface radiation budget from pole to pole. The latest flux estimation techniques have been used along with data from the International Satellite Cloud Climatology Project and the Earth Radiation Budget Experiment. Regional, zonal, and hemispheric distributions of the downward and net components of both shortwave and longwave fluxes and of the net total surface fluxes are determined. Seasonal flux variation per region, expressed as flux range, is illustrated for these components also. The estimated fluxes appear to be accurate to about 16 W/sq m on a global average, based on sensitivity analyses and comparisons with ground data. An analysis of flux errors showed that most of the error was attributable to errors in input data. 22 refs.
Article
In this study, numerical simulations of the twentieth-century climate are evaluated, focusing on the changes in the intensity of the global water cycle. A new model diagnostic of atmospheric water vapor cycling rate is developed and employed that relies on constituent tracers predicted at the model time step. This diagnostic is compared to a simplified traditional calculation of cycling rate, based on monthly averages of precipitation and total water content. The mean sensitivity of both diagnostics to variations in climate forcing is comparable. However, the new diagnostic produces systematically larger values with more variability.Climate simulations were performed using SSTs of the early (1902-21) and late (1979-98) twentieth century along with the appropriate CO2 forcing. In general, the increase of global precipitation with the increases in SST that occurred between the early and late twentieth century is small. However, an increase of atmospheric temperature leads to a systematic increase in total precipitable water. As a result, the residence time of water in the atmosphere increased, indicating a reduction of the global cycling rate. This result was explored further using a number of 50-yr climate simulations from different models forced with observed SST. The anomalies and trends in the cycling rate and hydrologic variables of different GCMs are remarkably similar. The global annual anomalies of precipitation show a significant upward trend related to the upward trend of surface temperature, during the latter half of the twentieth century. While this implies an increase in the simulated hydrologic cycle intensity, a concomitant increase of total precipitable water again leads to a decrease in the calculated global cycling rate. An analysis of the land/sea differences shows that the simulated precipitation over land has a decreasing trend, while the oceanic precipitation has an upward trend consistent with previous studies and the available observations. The decreasing continental trend in precipitation is located primarily over tropical land regions, with some other regions, such as North America, experiencing an increasing trend. Precipitation trends are diagnosed further using the water tracers to delineate the precipitation that occurs because of continental evaporation, as opposed to oceanic evaporation. These model diagnostics show that over global land areas, the recycling of continental moisture is decreasing in time. However, the recycling changes are not spatially uniform so that some regions, most notably over the United States, experience continental recycling of water that increases in time.
Article
Long-term in situ observations are widely used in a variety of climate analyses. Unfortunately, most decade- to century-scale time series of atmospheric data have been adversely impacted by inhomogeneities caused by, for example, changes in instrumentation, station moves, changes in the local environment such as urbanization, or the introduction of different observing practices like a new formula for calculating mean daily temperature or different observation times. If these inhomogeneities are not accounted for properly, the results of climate analyses using these data can be erroneous. Over the last decade, many climatologists have put a great deal of effort into developing techniques to identify inhomogeneities and adjust climatic time series to compensate for the biases produced by the inhomogeneities. It is important for users of homogeneity-adjusted data to understand how the data were adjusted and what impacts these adjustments are likely to make on their analyses. And it is important for developers of homogeneity-adjusted data sets to compare readily the different techniques most commonly used today. Therefore, this paper reviews the methods and techniques developed for homogeneity adjustments and describes many different approaches and philosophies involved in adjusting in situ climate data.
Article
A modified method for deriving free-water evaporation estimates from network observations of air temperature, dew point, wind movement, and incoming minus reflected radiation is presented. Taking into account the difference between air and water temperature in computing emitted radiation from the water surface, the expression is an improvement over the original Penman type equation where observation of net radiation over the actual water surface is lacking. The accuracy of the method depends on the applicable mass transfer wind function. Techniques are derived to adjust for the effects of advected energy and heat storage when applying the free-water evaporation estimate to actual water bodies. Computations of lake evaporation made with the modified method for a number of locations where verification data are available indicate that the relation provides a suitable basis for estimating actual evaporation without the expense of continuous over-water observations.
Article
The Class A pan evaporation rates at many Australian observing stations have reportedly decreased between 1970 and 2002. That pan evaporation rates have decreased at the same time that temperatures have increased has become known as the “pan evaporation paradox.” Pan evaporation is primarily dependant on relative humidity, solar radiation, and wind. In this paper, trends in observed pan evaporation in Australia during the period 1975–2004 were attributed to changes in other climate variables using a Penman-style pan evaporation model. Trends in daily average wind speed (termed wind run) were found to be an important cause of trends in pan evaporation. This result is a significant step toward resolving the pan evaporation paradox for Australia. Data inspection and interstation comparison revealed that some of the significant wind run trends were discontinuous or spatially uncorrelated. These analyses raised the possibility that some of the changes in observed wind run, and by implication some of the significant changes in pan evaporation, may represent changes in the local environment surrounding the observing stations. Daily pressure gradients and NCEP–NCAR reanalysis wind surfaces were analyzed in an attempt to identify any climatological wind run trends associated with large-scale changes in atmospheric circulations. Unfortunately, the trends from the two data sources were not consistent, and the challenge remains to conclusively identify the cause or causes of the changes in observed station wind run in Australia.
Article
Two theoretical approaches to evaporation from saturated surfaces are outlined, the first being on an aerodynamic basis in which evaporation is regarded as due to turbulent transport of vapour by a process of eddy diffusion, and the second being on an energy basis in which evaporation is regarded as one of the ways of degrading incoming radiation. Neither approach is new, but a combination is suggested that eliminates the parameter measured with most difficulty-surface temperature-and provides for the first time an opportunity to make theoretical estimates of evaporation rates from standard meteorological data, estimates that can be retrospective. Experimental work to test these theories shows that the aerodynamic approach is not adequate and an empirical expression, previously obtained in America, is a better description of evaporation from open water. The energy balance is found to be quite successful. Evaporation rates from wet bare soil and from turf with an adequate supply of water are obtained as fractions of that from open water, the fraction for turf showing a seasonal change attributed to the annual cycle of length of daylight. Finally, the experimental results are applied to data published elsewhere and it is shown that a satisfactory account can be given of open water evaporation at four widely spaced sites in America and Europe, the results for bare soil receive a reasonable check in India, and application of the results for turf shows good agreement with estimates of evaporation from catchment areas in the British Isles.
Article
Utilizing the scenario development framework from Mahmoud et al. (2009), a set of scenarios were developed for and applied in the Verde River Watershed in Arizona, USA. Through a scenario definition exercise, three dimensions of future change with respective axis extremes were identified: climate change (periodic droughts vs. sustained drought), demographics (water-conservative population vs. water-consumptive population), and the economy (booming economy vs. poor economy). In addition to the various combinations of dimension extremes, each scenario was given a unique event or theme that was characteristic of the combination of dimension extremes it possessed. The scenarios were then fleshed out into narrative forms that expanded on the details of each scenario’s internal temporal evolution. The scenarios were analyzed by a water supply and demand model that was specifically constructed for their simulation. Following the analysis of scenario results, assessment narratives were provided to outline the impact of each scenario on the Verde River Watershed and management operations in that basin.
Article
We present the longest data set of observed soil moisture available in the world, 45 yr of gravimetrically-observed plant available soil moisture for the top 1 m of soil, observed every 10 days for April–October for 141 stations from fields with either winter or spring cereals from the Ukraine for 1958–2002. We averaged the summer observations over the entire region to account for the observed scale of soil moisture variations, to enhance the portion of the variance that is related to meteorological forcing. The observations show a positive soil moisture trend for the entire period of observation, with the trend leveling off in the last two decades. Although models of global warming predict summer desiccation in a greenhouse-warmed world, there is no evidence for this in the observations yet, even though the region has been warming for the entire period. While the interannual variations of soil moisture simulated by both the European Centre for Medium-range Weather Prediction and the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalyses are close to the observations, neither reanalysis simulates the observed upward trend. Climate model simulations for the period show the same general shape as the observations, but differ quite a bit from each other and from the observations. An observed downward trend in insolation may have produced a downward trend in evaporation and may have contributed to the upward soil moisture trend.
Article
The water balance and hydrologic characteristics of streamflow and subsurface outflow were measured for a first-order catchment draining a 23.4-ha stand of undisturbed terra firme rain forest in the central Amazon Basin over a 1 year period. Total rainfall during the study period, representing a 1 in 10 wet year, was 2870 mm of which 1650 mm and 42 mm respectively were exported from the catchment as streamflow and subsurface outflow. A hydrograph separation technique applied to the entire streamflow record distinguished 173 storms. The storm flow volume was 88 mm, which represented only 5% of overall streamflow and less than 3% of the rainfall. The 20% of least frequent, large-volume storms accounted for 75% of the rainfall volume and 80% of the storm flow volume. The fraction of the catchment that could account for the volume of storm flow runoff in a given storm as overland flow never exceeded 4%, even for the largest storms. The change in soil water storage from the previous year was equivalent to about 57 mm of water. Evapotranspiration, estimated as the water balance residual, was equivalent to 1120 mm per year with an uncertainty, estimated by propagating measurement errors, of ±16–21%. Evapotranspiration is significantly less than values previously reported from catchment-scale water balance studies in the central Amazon, and is slightly lower than recent estimates derived from micrometeorologieal techniques, though differences in the latter case are within measurement error.
Article
Mean annual evapotranspiration from a catchment is determined largely by precipitation and potential evapotranspiration; characteristics of the catchment (e.g., soil, topography, etc.) play only a secondary role. It has been shown that the ratio of mean annual potential evapotranspiration to precipitation (referred as the index of dryness) can be used to estimate mean annual evapotranspiration by using one additional parameter. This study evaluates the effects of climatic and catchment characteristics on the partitioning of mean annual precipitation into evapotranspiration using a rational function approach, which was developed based on phenomenological considerations. Over 470 catchments worldwide with long-term records of precipitation, potential evapotranspiration, and runoff were considered, and results show that model estimates of mean annual evapotranspiration agree well with observed evapotranspiration taken as the difference between precipitation and runoff. The mean absolute error between modeled and observed evapotranspiration was 54 mm, and the model was able to explain 89% of the variance with a slope of 1.00 through the origin. This indicates that the index of dryness is the most significant variable in determining mean annual evapotranspiration. Results also suggest that forested catchments tend to show higher evapotranspiration than grassed catchments and their evapotranspiration ratio (evapotranspiration divided by precipitation) is most sensitive to changes in catchment characteristics for regions with the index of dryness around 1.0. Additionally, a stepwise regression analysis was performed for over 270 Australian catchments where detailed information of vegetation cover, precipitation characteristics, catchment slopes, and plant available water capacity was available. It is shown that apart from the index of dryness, average storm depth, plant available water capacity, and storm arrival rate are also significant.
Article
Results from bivariate linear regression indicate that strong associations exist between height and surface data for all seasons except winter. The influence of persistent snow cover and topography may have a major influence on low wintertime associations. Trends in 700 mb height data are significantly correlated to seasonal temperature trends as measured by Spearman rank correlation. These results suggest that upper air trends are consistent with and supportive of the trends observed at the surface, and so lend credence to surface trend existence. -from Authors
Article
The amount of solar irradiance reaching the surface is a key parameter in the hydrological and energy cycles of the Earth's climate. We analyze 20th Century simulations using nine state-of-the-art climate models and show that all models estimate a global annual mean reduction in downward surface solar radiation of 1-4 W/m2 at the same time that the globe warms by 0.4-0.7°C. In single forcing simulations using the GISS-ER model, this ``global dimming'' signal is shown to be predominantly related to aerosol effects. In the global mean sense the surface adjusts to changes in downward solar flux instantaneously by reducing the upward fluxes of longwave, latent and sensible heat. Adding increased greenhouse gas forcing traps outgoing longwave radiation in the atmosphere and surface which results in net heating (although reduced) that is consistent with global warming over the 20th Century. Over the 1984-2000 period, individual model simulations show widely disparate results, mostly related to cloud changes associated with tropical Pacific variations, similar to the changes inferred from the satellite data analysis. This suggests that this time period is not sufficient to determine longer term trends.
Article
An idea to use a simple convective boundary layer (CBL) model in the complementary relationship to estimate regional evaporation was explored. The CBL model simulated the potential specific humidity deficit D in CBL when the bulk stomatal resistance rst=0. This value of D was then used in the Penman-type equation to derive evaporation Epo that would occur with ample soil moisture under the prevailing weather condition. The same equation was also used to produce the potential evaporation Ep with the actual humidity deficit, and these Ep and Epo values allowed the evaluation of the actual evaporation E by applying the complementary relationship E=εEpo-Ep, where ε is assumed as 2.0. This was tested with the data set obtained in Hexi Corridor desert area in northwestern China with a modified version of a simple CBL model developed by Lhomme [1997]. It was found that the method produced better estimates of the daytime mean E values with smaller bias than those obtained from a conventional application of the complementary relationship without the CBL model. Also, it was shown that the assumption of ε=2.0 in the complementary relationship was only approximate in most cases. To take this into account, an additional procedure was explored in which ε was treated as a variable, and an iteration process with the CBL model determined the final ε and E values. It was found that this process produced E values that have smaller systematic error and agree better with the measurements on average, but the unsystematic error got worse than that found with ε=2.0, probably because of use of the CBL model with rst!=0 in the iteration process.