Gerald N. Flerchinger

Intercode comparisons for simulating water balance of surficial sediments in semiarid regions

[1] Near-surface water balance modeling is often used to evaluate land-atmosphere interactions, deep drainage, and groundwater recharge. The purpose of this study was to compare water balance simulation results from seven different codes, HELP, HYDRUS1D, SHAW, SoilCover, SWIM, UNSAT-H, and VS2DTI, using 1–3 year water balance monitoring data from nonvegetated engineered covers (3 m deep) in warm (Texas) and cold (Idaho) desert regions. Simulation results from most codes were similar and reasonably approximated measured water balance components. Simulation of infiltrationexcess runoff was a problem for all codes. Annual drainage was estimated to within ±64% by most codes. Outliers result from the modeling approach (storage routing versus Richards’ equation), upper boundary condition during precipitation, lower boundary condition (seepage face versus unit gradient), and water retention function (van Genuchten versus Brooks and Corey). A unique aspect of the code comparison study was the ability to explain the outliers by incorporating the simulation approaches (boundary conditions or hydraulic parameters) used in the outlying codes in a single code and comparing the results of the modified and unmodified code. This approach overcomes the criticism that valid code comparisons are infeasible because of large numbers of differences among codes. The code comparison study identified important factors for simulating the near-surface water balance. INDEX TERMS: 1866 Hydrology: Soil moisture; 1818 Hydrology: Evapotranspiration; 1875 Hydrology: Unsaturated zone; 1836 Hydrology: Hydrologic budget (1655); KEYWORDS: hydrologic budget, soil moisture, unsaturated zone, water balance modeling

Modeling Evapotranspiration and Surface Energy Budgets Across a Watershed

Transport of mass and energy between and within soils, canopies, and the atmosphere is an area of increasing interest in hydrology and meteorology. On arid and semiarid rangelands, evapotranspiration (ET) can account for over 90% of the precipitation, making accurate knowledge of the surface energy balance particularly critical. Recent advances in measurement and modeling have made the accurate estimate of ET and the entire surface energy balance possible. The Simultaneous Heat and Water (SHAW) model, a detailed physical process model capable of simulating the effects of a multispecies plant canopy on heat and water transfer, was applied to 2 years of data collected for three vegetation types (low sagebrush, mountain big sagebrush, and aspen) on a semiarid watershed. Timing and magnitude of ET from the three sites differed considerably. Measured and simulated ET for approximately 26 days of measurement in 1990 were 41 and 44 mm, respectively, for the low sagebrush, 74 and 69 mm for the mountain big sagebrush, and 85 and 89 mm for the aspen. Simulated and measured cumulative ET for up to 85 days of measurement at the three sites in 1993 differed by 3–5%. Simulated diurnal variation in each of the surface energy balance components compared well with measured values. Model results were used to estimate total ET from the watershed as a basis for a complete water budget of the watershed.

A ten-year water balance of a mountainous semi-arid watershed

Quantifying water balance components, which is particularly challenging in snow-fed, semi-arid regions, is crucial to understanding the basic hydrology of a watershed. In this study, a water balance was computed using 10 years of data collected at the Upper Sheep Creek Watershed, a 26-ha semi-arid mountainous sub-basin within the Reynolds Creek Experimental Watershed in southwest Idaho, USA. The approach computed a partial water balance for each of three landscape units and then computed an aggregated water balance for the watershed. Runoff and change in ground water storage were not distinguishable between landscape units. Precipitation, which occurs predominantly as snow, was measured within each landscape unit directly and adjusted for drifting. Spatial variability of effective precipitation was shown to be greater during years with higher precipitation. Evapotranspiration, which accounted for nearly 90% of the effective precipitation, was estimated using the Simultaneous Heat and Water (SHAW) Model and validated with measurements from Bowen ratio instruments. Runoff from the watershed was correlated to precipitation above a critical threshold of approximately 450 mm of precipitation necessary to generate runoffOr 2 a 0:52U: The average water balance error was 46 mm, or approximately 10% of the estimated effective precipitation for the ten-year period. The error was largely attributed to deep percolation losses through fractures in the basalt underlying the watershed. Simulated percolation of the water beyond the root zone correlated extremely well with measured runoffOr 2 a 0:90U; which is derived almost entirely from subsurface flow. Above a threshold of 50 mm, approximately 67% of the water percolating beyond the root zone produces runoff. The remainder was assumed to be lost to deep percolation through the basalt. This can have important ramifications in addressing subsurface flow and losses when applying a snowmelt runoff model to simulate runoff and hydrologic processes in the watershed. q 2000 Published by Elsevier Science B.V.

Thirty-five years of research data collection at the Reynolds Creek Experimental Watershed, Idaho, United States

Comprehensive, long-term hydrologic data sets for watershed systems are valuable for hydrologic process research; for interdisciplinary ecosystem analysis; for model development, calibration, and validation; and for assessment of change over time. The Reynolds Creek Experimental Watershed in southwestern Idaho, United States, was established in 1960 and provides a research facility and comprehensive long-term database for science. Spatial data layers for terrain, soils, geology, vegetation, and basic site mapping features and databases for fundamental hydrologic parameters of precipitation, snow, climate, soil microclimate, and stream discharge and sediment concentration are now available for water years 1962-1996 and are described in the following eight data reports.

Application of a soil-water balance model to evaluate the influence of Holocene climate change on calcic soils, Mojave Desert, California, U.S.A.

Abstract We used a process-based soil-water balance model to simulate the downward flux of soil-water under varied conditions of climate, vegetation, and soil texture to determine the potential impact of episodic periods of wetter (pluvial) climate during the Holocene on calcic soils in the Mojave Desert that have a bimodal distribution of carbonate. Daily weather data associated with a relatively “wet” climate (years with extreme increases in annual rainfall, ∼ 33 cm/yr) and “dry” climate (historic average annual rainfall, ∼ 15 cm/yr) was used to simulate the affects of Pleistocene and Holocene climate change on soil-water balance. Linkages among atmospheric circulation patterns, regional increases in precipitation, and historic flooding in the Mojave Desert, California, suggest that historic wet years provide an analog for wetter climates that occurred during the last glacial period (latest Pleistocene) and episodically during Holocene periods of pluvial activity. Modeling results indicate that soil-water balance for dry and wet years strongly corresponds with the upper and lower zones of carbonate accumulation respectively. Soil-water only reached the lower zone of carbonate during a wet year when extreme increases in winter/spring storm activity resulted in a significant increase in precipitation and the downward flux of soil water. The linkage between increases in frontal storm activity and pluvial events suggests that the shallow zone of the bimodal distribution of carbonate is a result of periods of significant decreases in winter and spring rainfall and not primarily due to increases in Holocene temperature or the development of clay-rich horizons. Calculation of carbonate solubility and accumulation rates suggests that the bimodal distribution of carbonates in soils may have also been impacted by episodic periods of extreme increases in precipitation associated with perennial lakes during the Holocene. Results suggests that much of the carbonate in the upper 75 cm of Pleistocene soils may have accumulated during the late Holocene rather than throughout the entire Holocene.

A unified approach for process-based hydrologic modeling: 2. Model implementation and case studies

This work advances a unified approach to process-based hydrologic modeling, which we term the “Structure for Unifying Multiple Modeling Alternatives (SUMMA).” The modeling framework, introduced in the companion paper, uses a general set of conservation equations with flexibility in the choice of process parameterizations (closure relationships) and spatial architecture. This second paper specifies the model equations and their spatial approximations, describes the hydrologic and biophysical process parameterizations currently supported within the framework, and illustrates how the framework can be used in conjunction with multivariate observations to identify model improvements and future research and data needs. The case studies illustrate the use of SUMMA to select among competing modeling approaches based on both observed data and theoretical considerations. Specific examples of preferable modeling approaches include the use of physiological methods to estimate stomatal resistance, careful specification of the shape of the within-canopy and below-canopy wind profile, explicitly accounting for dust concentrations within the snowpack, and explicitly representing distributed lateral flow processes. Results also demonstrate that changes in parameter values can make as much or more difference to the model predictions than changes in the process representation. This emphasizes that improvements in model fidelity require a sagacious choice of both process parameterizations and model parameters. In conclusion, we envisage that SUMMA can facilitate ongoing model development efforts, the diagnosis and correction of model structural errors, and improved characterization of model uncertainty.

Comparing Simulated and Measured Sensible and Latent Heat Fluxes over Snow under a Pine Canopy to Improve an Energy Balance Snowmelt Model

Abstract During the second year of the NASA Cold Land Processes Experiment (CLPX), an eddy covariance (EC) system was deployed at the Local Scale Observation Site (LSOS) from mid-February to June 2003. The EC system was located beneath a uniform pine canopy, where the trees are regularly spaced and are of similar age and height. In an effort to evaluate the turbulent flux calculations of an energy balance snowmelt model (SNOBAL), modeled and EC-measured sensible and latent heat fluxes between the snow cover and the atmosphere during this period are presented and compared. Turbulent fluxes comprise a large component of the snow cover energy balance in the premelt and ripening period (March–early May) and therefore control the internal energy content of the snow cover as melt accelerates in late spring. Simulated snow cover depth closely matched measured values (RMS difference 8.3 cm; Nash–Sutcliff model efficiency 0.90), whereas simulated snow cover mass closely matched the few measured values taken during...

Groundwater response to snowmelt in a mountainous watershed

Abstract Snowmelt recharge to shallow groundwater systems is the primary source of streamflow in many mountainous watersheds, but characteristics of these systems are not well understood, and their contribution to streamflow is often not appreciated. Data from a detailed study on the Upper Sheep Creek Watershed located within the Reynolds Creek Experimental Watershed in southwestern Idaho were analyzed to characterize the interactions between snowmelt, groundwater and streamflow. Response time between snowmelt, groundwater levels and streamflow was drastically different from year to year depending on the extent of the snowpack. Response time to snowmelt for piezometer and weirs located 135 m downslope from am isolated drift was 3–5 days during an average snow year and up to 70 days for a year with snow accumulation that was 40% of normal. The primary aquifer is believed to be unconfined during low snowmelt years and confined when normal or above-normal snowmelt causes high groundwater levels. Snowmelt from an isolated drift enters the primary aquifer upslope of the confining layer. Rapid response during years with normal snow accumulation is therefore primarily a pressure pulse through the confined aquifer. Recharge during years of low snow accumulation is insufficient to fill the primary aquifer to the confining layer,and response time is indicative of travel time through the aquifer.

Effects of crop residue cover and architecture on heat and water transfer at the soil surface

Different residue types and standing stubble versus distributed flat residues affect heat and water transfer at the soil surface to varying degrees. Understanding the effects of various residue configurations can assist in better residue management decisions, but this is complex due to various interacting influences. Therefore, modeling the effects of crop residues on heat and water movement can be an effective tool to assess the benefits of differing residues types and architectures for various climates. The purpose of this study was to test the ability of the Simultaneous Heat And Water (SHAW) model for simulating the effects of residue type and architecture on heat and water transfer at the surface and to evaluate the impacts of differing residue types and architectures on heat and water transfer in significantly different climates. The model was tested on bare tilled soils and corn, wheat and millet residues having varying amounts of standing and distributed flat residues for three separate locations: Ames, IA, Akron, CO and Pullman, WA. Modifications to the model were necessary to correctly simulate the effect of wind on convective transfer through a flat corn residue layer. Model efficiencies for simulated soil temperature approached or exceeded 0.90 for nearly all residue treatments and locations. The root mean square deviation for simulated water content compared to measured values was typically around 0.04 m3 m−3. Satisfied that the model could reasonably simulate the effect of residue type and architecture, the model was applied to simulate the effects of differing residue architectures to 30 years of generated weather conditions for four diverse climate stations: Boise, ID; Spokane, WA; Des Moines, IA; Minneapolis, MN. Simulated frost depths for bare and standing residues were typically deeper than for flat residues. Bare soil had the highest evaporation at all sites, and flat wheat residue generally had the lowest evaporation. The wetter climates (Des Moines and Minneapolis) tended to favor flat residues for reducing evaporation more so than the drier climates. Near-surface soil temperature under standing residues warmed to 5 °C in the spring by as much as 5–9 days earlier compared to bare and flat residue cover depending on location, which can have important ramifications for early seedling germination and plant establishment.

Long‐Term Soil Water Content Database, Reynolds Creek Experimental Watershed, Idaho, United States

We describe long-term soil water data collected at the Reynolds Creek Experimental Watershed (RCEW). Data were collected for 10 -25 years at 18 sites representing different climatic regimes and soils in the RCEW. Soil profile data are also available. High correlation between neutron probe and lysimeter measurements are the basis for assessing the accuracy of neutron probe-measured changes in soil water content. These data are available to the public via the U.S. Department of Agriculture, Agricultural Research Service, Northwest Watershed Research Center anonymous ftp site ftp.nwrc.ars.usda.gov.