Gautam Bisht

Pacific Northwest National Laboratory | PNNL · Atmospheric Sciences and Global Change Division

Understanding the influence of land surface heterogeneity on surface water and energy fluxes is crucial for modeling earth system variability and change. This study investigates the effects of four dominant heterogeneity sources on land surface modeling, including atmospheric forcing (ATM), soil properties (SOIL), land use and land cover (LULC), an...

Runoff is a critical component of the terrestrial water cycle, and Earth system models (ESMs) are essential tools to study its spatiotemporal variability. Runoff schemes in ESMs typically include many parameters so that model calibration is necessary to improve the accuracy of simulated runoff. However, runoff calibration at a global scale is chall...

Topographic heterogeneity and lateral subsurface flow at the hillslope scale of ≤ 1 km may have outsized impacts on tropical forest through their impacts on water available to plants under water stressed conditions. However, vegetation dynamics and finer‐scale hydrologic processes are not concurrently represented in Earth system models. In this stu...

With the highest albedo of the land surface, snow plays a vital role in Earth’s surface energy and water cycles. Snow albedo is greatly affected by snow grain properties (e.g., size and shape) and light absorbing particles (LAPs) such as black carbon (BC) and dust. The mixing state of LAPs in snow also has large impacts on LAP-induced snow albedo r...

Sub-grid topographic heterogeneity has large impacts on surface energy balance and land-atmosphere interactions. However, the impacts of representing sub-grid topographic effects in land surface models (LSMs) on surface energy balance and boundary conditions remain unclear. This study analyzed and evaluated the impacts of sub-grid topographic repre...

The Earth's land surface features spatial and temporal heterogeneity over a wide range of scales below those resolved by current Earth system models. State-of-the-art land and atmosphere models employ parameterizations to represent their subgrid heterogeneity, but the land-atmosphere coupling in ESMs typically operates on the grid scale. Communicat...

Watershed delineation and flow direction representation are the foundations of streamflow routing in spatially distributed hydrologic modeling. A recent study showed that hexagon-based watershed discretization has several advantages compared to the traditional Cartesian (latitude-longitude) discretization, such as uniform connectivity and compatibi...

In tropical forests, both vegetation characteristics and soil properties are important not only for controlling energy, water, and gas exchanges directly but also determining the competition among species, successional dynamics, forest structure and composition. However, the joint effects of the two factors have received limited attention in Earth...

Runoff is a critical component of the terrestrial water cycle and Earth System Models (ESMs) are essential tools to study its spatio-temporal variability. Runoff schemes in ESMs typically include many parameters so model calibration is necessary to improve the accuracy of simulated runoff. However, runoff calibration at global scale is challenging...

Topography exerts significant influences on the incoming solar radiation at the land surface. A few stand-alone regional and global atmospheric models have included parameterizations for sub-grid topographic effects on solar radiation. However, nearly all Earth system models (ESMs) that participated in the Coupled Model Intercomparison Project (CMI...

Topography exerts significant influences on the incoming solar radiation at the land surface. A few stand-alone regional and global atmospheric models have included parameterizations for sub-grid topographic effects on solar radiation. However, nearly all Earth System Models (ESMs) that participated in the Coupled Model Intercomparison Project (CMI...

The Community Land Model (CLM) is an effective tool to simulate the biophysical and biogeochemical processes and their interactions with the atmosphere. Although CLM version 5 (CLM5) constitutes various updates in these processes, its performance in simulating energy, water and carbon cycles over the contiguous United States (CONUS) at scales which...

Water exchange between the surface and subsurface is important for both water resource management and environmental protection. In this paper, we develop coupled surface and subsurface flow simulation capability in a parallel subsurface flow and reactive transport code PFLOTRAN. We sequentially couple the diffusion wave-based surface flow with the...

Abstract This paper documents the biogeochemistry configuration of the Energy Exascale Earth System Model (E3SM), E3SMv1.1‐BGC. The model simulates historical carbon cycle dynamics, including carbon losses predicted in response to land use and land cover change, and the responses of the carbon cycle to changes in climate. In addition, we introduce...

Abstract In this study, we use the Community Land Model Version 5 (CLM5) to investigate how irrigation modulates hydrologic and biogeochemical dynamics in the Upper Columbia‐Priest Rapids (UCPR) watershed, a typical semiarid watershed located in the northwestern United States dominated by cropland. To our knowledge, this constitutes the first appli...

The Community Land Model (CLM) is the land component of the Community Earth System Model (CESM) and is used in several global and regional modeling systems. In this paper, we introduce model developments included in CLM version 5 (CLM5), which is the default land component for CESM2. We assess an ensemble of simulations, including prescribed and pr...

Abstract Evapotranspiration (ET) plays an important role in land‐atmosphere coupling of energy, water, and carbon cycles. Following deforestation, ET is typically observed to decrease substantially as a consequence of decreases in leaf area and roots and increases in runoff. Changes in ET (latent heat flux) revise the surface energy and water budge...

Abstract This work documents the first version of the U.S. Department of Energy (DOE) new Energy Exascale Earth System Model (E3SMv1). We focus on the standard resolution of the fully coupled physical model designed to address DOE mission‐relevant water cycle questions. Its components include atmosphere and land (110‐km grid spacing), ocean and sea...

Abstract Over the past several decades, the land modeling community has recognized the importance of nutrient regulation on the global terrestrial carbon cycle. Implementations of nutrient limitation in land models are diverse, varying from applying simple empirical down‐regulation of potential gross primary productivity under nutrient deficit cond...

Abstract Current generation Land Surface Models (LSMs) routinely simulate many nonlinear multiphysics processes. The complexity of future generation LSMs is expected to increase as critical new biophysical and biogeochemical processes are incorporated. Current generation LSMs have several shortcomings including the lack of robust numerical methods...

Improving global-scale model representations of near-surface soil moisture and groundwater hydrology is important for accurately simulating terrestrial processes and predicting climate change effects on water resources. Most existing land surface models, including the default E3SM Land Model (ELMv0), which we modify here, routinely employ different...

Quantitative understanding of controls on thaw layer thickness (TLT) dynamics in the Arctic peninsula is essential for predictive understanding of permafrost degradation feedbacks to global warming and hydrobiochemical processes. This study jointly interprets electrical resistivity tomography (ERT) measurements and hydro-thermal numerical simulatio...

Improving global-scale model representations of coupled surface and groundwater hydrology is important for accurately simulating terrestrial processes and predicting climate change effects on water resources. Most existing land surface models, including the default E3SM Land Model (ELMv0), which we modify here, routinely employ different formulatio...

Redox gradients within hyporheic zones significantly impact the biogeochemical cycling of carbon and nitrogen. To investigate the effect of these redox gradients on nitrogen transformation in the subsurface, we integrated a genome-informed complex reaction network into PFLOTRAN, which is an open source, massively parallel, three-dimensional, reacti...

A fully coupled three-dimensional surface and subsurface land model is developed and applied to a site along the Columbia River to simulate three-way interactions among river water, groundwater, and land surface processes. The model features the coupling of the Community Land Model version 4.5 (CLM4.5) and a massively parallel multiphysics reactive...

Microtopographic features, such as polygonal ground, are characteristic sources of landscape heterogeneity in the Alaskan Arctic coastal plain. Here, we analyze the effects of snow redistribution (SR) and lateral subsurface processes on hydrologic and thermal states at a polygonal tundra site near Barrow, Alaska. We extended the land model integrat...

A fully coupled three-dimensional surface and subsurface land model is developed and applied to a site along the Columbia River to simulate three-way interactions among river water, groundwater, and land surface processes. The model features the coupling of the Community Land Model version 4.5 (CLM4.5) and a massively-parallel multi-physics reactiv...

Redox gradients within hyporheic zones significantly impact the biogeochemical cycling of carbon and nitrogen. To investigate the effect of these redox gradients on nitrogen transformation in the subsurface, we integrated a genome-informed complex reaction network into PFLOTRAN, which is an open source, massively parallel, three-dimensional, reacti...

Vast carbon stocks stored in permafrost soils of Arctic tundra are under risk of release to the atmosphere under warming climate scenarios. Ice-wedge polygons in the low-gradient polygonal tundra create a complex mosaic of microtopographic features. This microtopography plays a critical role in regulating the fine-scale variability in thermal and h...

We explore coupling to a configurable subsurface reactive transport code as a flexible and extensible approach to biogeochemistry in land surface models. A reaction network with the Community Land Model carbon–nitrogen (CLM-CN) decomposition, nitrification, denitrification, and plant uptake is used as an example. We implement the reactions in the o...

Vast carbon stocks stored in permafrost soils of Arctic tundra are under risk of release to atmosphere under warming climate. Ice-wedge polygons in the low-gradient polygonal tundra create a complex mosaic of microtopographic features. The microtopography plays a critical role in regulating the fine scale variability in thermal and hydrological reg...

We explore coupling to a configurable subsurface reactive transport code as a flexible and extensible approach to biogeochemistry in land surface models; our goal is to facilitate testing of alternative models and incorporation of new understanding. A reaction network with the CLM-CN decomposition, nitrification, denitrification, and plant uptake i...

Predictive understanding of northern peatland hydrology is a necessary precursor to understanding the fate of massive carbon stores in these systems under the influence of present and future climate change. Current models have begun to address microtopographic controls on peatland hydrology, but none have included a prognostic calculation of peatla...

Biogeochemical gradients that exist within the hyporheic zone play a key role in cycling carbon, both organic and inorganic. To investigate the impact of various processes on the fate and transport of carbon in the hyporheic zone, we develop a biogeochemical reaction network and integrate it into the flow and reactive transport code PFLOTRAN. The m...

Predictive understanding of northern peatland hydrology is a necessary precursor to understanding the fate of massive carbon stores in these systems under the influence of present and future climate change. Current models have begun to address microtopographic controls on peatland hydrology, but none have included a prognostic calculation of peatla...

High-resolution predictions of land surface hydrological dynamics are desirable for improved investigations of regional- and watershed-scale processes. Direct deterministic simulations of fine-resolution land surface variables present many challenges, including high computational cost. We therefore propose the use of reduced-order modeling techniqu...

Microtopographic features, such as polygonal ground, are characteristic sources of landscape heterogeneity in the Alaskan Arctic coastal plain. Here, we analyze the hypothesis that microtopography is a dominant controller of soil moisture in polygonal landscapes. We perform multi-year surface–subsurface isothermal flow simulations using the PFLOTRA...

Existing land surface models (LSMs) describe physical and biological processes that occur over a wide range of spatial and temporal scales. For example, biogeochemical and hydrological processes responsible for carbon (CO2, CH4) exchanges with the atmosphere range from the molecular scale (pore-scale O2 consumption) to tens of kilometers (vegetatio...

Existing land surface models (LSMs) describe physical and biological processes that occur over a wide range of spatial and temporal scales. For example, biogeochemical and hydrological processes responsible for carbon (CO2, CH4) exchanges with the atmosphere range from the molecular scale (pore-scale O2 consumption) to tens of kilometers (vegetatio...

Arctic and sub-Arctic soils currently contain approximately 1700 billion metric tones of frozen organic carbon, approximately 200 times current annual anthropogenic emissions. This carbon is vulnerable to release to the atmosphere as CO 2 and CH 4 as high-latitude temperatures warm. The NGEE-Arctic project is working to optimally inform process rep...

Arctic and sub-Arctic soils store vast amounts of carbon, approximately 1700 billion metric tones of frozen organic carbon. This carbon is susceptible to release to the atmosphere due to environmental changes (e.g., rapidly evolving landscape, warming); however, the mechanisms responsible for this susceptibility of soil organic matter (SOM) are not...

We present an overview of the recent results from the Oak Ridge National Laboratory land modeling team toward the Climate Science for a Sustainable Energy Future (CSSEF) project. One of the goals of the CSSEF project is to improve and accelerate the climate model development process leading toward the 6th generation of the Community Earth System Mo...

Numerous studies have shown a positive soil moisture-rainfall feedback through observational data, as well as, modeling studies. Spatial variability of topography, soils, and vegetation play a significant role in determining the response of land surface states (soil moisture) and fluxes (runoff, evapotranspirtiaon); but their explicit accounting wi...

Net radiation, at the Earth's surface, is a key variable of interest in fields such as hydrology, climate research, and agriculture. Retrieval algorithms for estimation of the surface radiation budget (SRB) from remote sensing data generally suffer from two major shortcomings: difficulty in dealing with cloudy-sky conditions and reliance on study-s...

Prior research has shown that deforestation in the southwestern Amazon enhances the formation of non-precipitating shallow cumulus clouds, while deep cumulus convection was favored over forested land. The research presented here further investigates the trends of hydrometeors in the area by examining how precipitation frequency changes as a functio...

The Fourth Assessment Report of the Intergovernmental Panel on Climate Change acknowledged that the lack of relevant observations in various regions of the world is a crucial gap in understanding and modeling impacts of climate change related to hydrologic cycle. The Surface Radiation Budget (SRB) is an important component in the study of land surf...

Net radiation is a key component in the surface radiation budget. Numerous studies have developed frameworks to estimate net radiation or its components (upwelling or downwelling longwave and/or shortwave radiation) from remote sensing data for clear sky conditions. Application of existing methodologies to estimate net radiation for cloudy sky cond...

Net radiation at the Earth's surface is a key component in the surface radiation budget (SRB) and necessary in fields such as hydrology, climate research, agriculture and renewable energy. Retrieval algorithms for estimating the SRB from satellite data generally suffer from two major shortcomings: difficulty in dealing with cloudy-sky conditions an...

Shallow clouds are prone to appear over deforested surfaces whereas deep clouds, much less frequent than shallow clouds, favor forested surfaces. Simultaneous atmospheric soundings at forest and pasture sites during the Rondonian Boundary Layer Experiment (RBLE-3) elucidate the physical mechanisms responsible for the observed correlation between cl...

A coupled, dynamic vegetation and hydrologic model, tRIBS+VEGGIE, was applied to the semiarid Walnut Gulch Experimental Watershed in Arizona. The physically-based, distributed nature of the coupled model allows for parameterization and simulation of watershed vegetation-water-energy dynamics on timescales varying from hourly to interannual. The mod...

Accurate initialization and representation of the land surface is of critical importance for increasing skill in regional weather and climate prediction. This paper presents a coupled land atmosphere model that emphasizes topographic control on partitioning of mass and energy fluxes at the land atmosphere boundary. The TIN (Triangulated Irregular N...

The goal of the proposed research is to enhance the predictability of hydrology and atmospheric conditions at daily, seasonal and inter-annual time scales. Capturing the interplay between seasonally dynamic vegetation and topography occurring through the local mechanisms of radiation and soil moisture re- distribution may contribute significantly t...

Soil moisture distributions are expected to be closely tied to ecosystem processes in water-limited environments of the southwest United States. Nevertheless, few studies have addressed how soil moisture varies across grassland to forest transitions frequently observed in semiarid mountain settings. In this study, we quantify the vegetation control...

A statistical analysis of ten years of remote sensing observations of cloudiness from geo-stationary satellites (GOES) has produced the strongest evidence of the impact of land cover over the deforested Amazon on the development of convective clouds. Shallow clouds are prone to appear over deforested surfaces while high clouds occur over forested s...

A fundamental problem in hydrologic science lies in understanding the interrelation among soil development, water and energy fluxes, and the mediating effect of vegetation in regulating the nature and extent of these processes. A semiarid basin in central New Mexico composed of three distinctive aspect slopes has been instrumented to study the ecoh...

Evapotranspiration (ET) cannot be measured directly from satellite observations but remote sensing can provide a reasonably good estimate of evaporative fraction (EF), defined as the ratio of ET and available radiant energy. It is feasible to estimate EF using a contextual interpretation of radiometric surface temperature (To) and normalized vegeta...

We present a new formulation to derive evaporative fraction (EF) and evapotranspiration (ET) maps from remotely sensed data without auxiliary relationships such as those relating a vegetation index and land surface temperature or site-specific relationships. The new equation is based on Grager and Gray's complementary relationship and Priestley-Tay...

The hydrometeorological conditions of mountain environments in semiarid regions are poorly understood, particularly during the North American Monsoon. Although it is well known that the climate and hydrology of mountain ranges are dynamically distinct of surrounding lowlands, little quantitative observational data has been collected to assess the s...