HYDROLOGIC BUDGET FOR A FRESHWATER MARSH IN FLORIDA1

ABSTRACT: Accurate water balance calculations are essential for water resource and environmental management decisions, but many of the terms used in the equation are difficult to measure. In this study, a method for measuring rates of evapotranspiration and net seepage from a freshwater marsh in southwest Florida is described. The results are compared to evaporation pan estimates as well as to calculations that balanced all the terms in the hydrologic budget. The measured rates of evapotranspiration showed a. distinct seasonal trend ranging from an average high of 0.24 in/d during July 1992 to a low of 0.06 in/d in January 1993. Evapotranspiration rates were higher than Class A evaporation pan measurements during July and August, indicating transpiration by plants exceeded evaporation by pans. Net ground water seepage flowed out of the marsh except during periods of high water table conditions. When all terms in the hydrologic budget were evaluated, the equation balanced on a yearly basis with an error of 2 percent, on a seasonal basis with errors less than 7 percent, but on a monthly basis errors were as great as 30 percent. Total annual rainfall on the marsh was 45 percent of the total marsh hydrologic input and was approximately equal to the loss by evapotranspiration of 41 percent.     

WATER YIELD IMPROVEMENT POTENTIAL BY VEGETATION MANAGEMENT ON WESTERN RANGELANDS1

Increasing water for onsite and offsite uses can be a viable objective for management of certain western rangelands. One approach utilizes water harvesting techniques to increase surface runoff by preventing or slowing infiltration of rain. An attractive alternative, where applicable, is to replace vegetation that uses much water with plants that use less so that more water percolates through the soil to streams and ground water. Most sites are too dry to increase water yield in this way; probably less than 1 percent of the western rangelands can be managed for this purpose. However, where annual precipitation exceeds about 450 mm (18 inches) and deep-rooted shrubs can be replaced by shallow-rooted grasses, there is potential to increase streamflows and to improve forage for livestock. Little or no increase can be expected by eradication of low-density brush and pinyon-juniper woodlands. Potentials for improving water yield are reviewed and summarized by vegetation types.     

APPLICATION OF CROP YIELD FUNCTIONS IN RESERVOIR OPERATION1

ABSTRACT: A model is developed for real-time operation of an irrigation reservoir with the objective of maximizing the value of multiple crop yields during a growing season. The model employs monthly additive and product forms of crop yield functions for dry matter and grain crops, respectively. The resulting nonlinear optimization model uses a log transform to reduce nonlinearities in the model. An application of the proposed model is compared to a common operating rule used in simulation models. The proposed model results were better in terms of net benefits from crop yields. The model uses GAMS (General Algebraic Modeling System) language. It requires an IBM-compatible microcomputer and is suitable for use by a reservoir manager.     

EFFECTS OF BASIN-SCALE TIMBER HARVEST ON WATER YIELD AND PEAK STREAMFLOW1

ABSTRACT: Streamflow changes resulting from clearcut harvest of lodgepole pine (Pinus contorta) on a 2145 hectare drainage basin are evaluated by the paired watershed technique. Thirty years of continuous daily streamflow records were used in the analysis, including 10 pre-harvest and 20 post-harvest years of data. Regression analysis was used to estimate the effects of timber harvest on annual water yield and annual peak discharge. Removal of 14 million board feet of lodgepole pine (Pinus contorta) from about 526 hectares (25 percent of the basin) produced an average of 14.7 cm additional water yield per year, or an increase of 52 percent. Mean annual daily maximum discharge also increased by 1.6 cubic meters per second or 66 percent. Increases occurred primarily during the period of May through August with little or no change in wintertime streamflows. Results suggest that clearcutting conifers in relatively large watersheds (> 2000 ha) may produce significant increases in water yield and flooding. Implications of altered streamflow regimes are important for assessing the future ecological integrity of stream ecosystems subject to large-scale timber harvest and other disturbances that remove a substantial proportion of the forest cover.     

FORMAL EDUCATION IN WILDLAND HYDROLOGY AND WATERSHED MANAGEMENT IN NORTH AMERICA1

ABSTRACT: A survey was made to determine the status of formal education in wildland hydrology by colleges and universities in the United States, Canada, and Mexico. As of December 1977 nine institutions offered the B.S. degree, 18 the M.S. degree, and 17 the degree of Ph.D. with a major or minor in watershed management, forest hydrology, or range hydrology. In addition, 8 other schools offer a minor in watershed management. The survey indicated 44 schools in the United States offer a total of 157 courses, five schools in Canada offer 24 courses, and 1 in Mexico offers one course in the related areas. The survey illustrated rapid growth in education programs and it is anticipated that growth will continue.     

Revisiting the Thornthwaite and Mather Water Balance1

Abstract: This technical note clarifies confusion currently present in research using and/or citing the 1955 and 1957 editions of the Thornthwaite and Mather Water Balance (or Budget) resulting from corrections to errant tables in the 1955 edition made by the authors for the 1957 edition. Confusion over the two editions and the formulas used have clouded the results of research, management, and educational publications ever since, resulting in frequent misunderstanding of the method’s utility as well as how its results compare with other frequently used methods for estimating evapotranspiration.     

MEASURING EVAPORATION WITH CERAMIC BELLANI PLATE ATMOMETERS1

ABSTRACT: Ceramic atmometers were tested to determine their usefulness for measuring evaporation in water resources applications. Field experiments were used to evaluate the precision, responsiveness to a range of potential evaporation conditions in a forested catchment, and interpretation of water loss of Bellani plate atmometers. The experiments, conducted from April to October in a warm, humid climate in the southeastern United States, indicate that atmometers can be reliable monitoring instruments for estimating potential evaporation. The small size, portability, low internal thermal mass, low cost, and ability to integrate the effect of radiation, air temperature, humidity and windspeed into one direct measurement of potential evaporation, make atmometers a useful instrument for certain water resources applications.     

Riparian Ground‐Water Flow Patterns Using Flownet Analysis: Evapotranspiration‐Induced Upwelling and Implications for N Removal1

Abstract: Ground‐water flow paths constrain the extent of nitrogen (N) sinks in deep, stratified soils of riparian wetlands. We examined ground‐water flow paths at four forested riparian wetlands in deep, low gradient, stratified deposits subjected to Southern New England’s temperate, humid climate. Mid‐day piezometric heads were recorded during the high water table period in April/May and again in late November at one site. Coupling field data with a two‐dimensional steady‐state ground‐water flow model, flow paths and fluxes were derived to 3 m depths. April/May evapotranspiration (ET) dominated total outflux (44‐100%) while flux to the stream was <10% of total outflux. ET exerted upward ground‐water flux through shallow carbon‐rich soils, increasing opportunities for N transformations and diverting flow from the stream. Dormant season results showed a marked increase in flux to the stream (27% of the total flux). Riparian sites with deep water tables (naturally or because of increased urbanization or other hydrologic modifications) or shallow root zones may not generate ground‐water upwelling to meet evaporative demand, thereby increasing the risk of N movement to streams. As water managers balance issues of water quality with water quantity, they will be faced with decisions regarding riparian management. Further work towards refining our understanding of ET mediation of N and water flux at the catchment scale will serve to inform these decisions.     

Hydrologic Calibration and Validation of SWAT in a Snow‐Dominated Rocky Mountain Watershed,Montana, U.S.A.1

Abstract: The Soil and Water Assessment Tool (SWAT) has been applied successfully in temperate environments but little is known about its performance in the snow‐dominated, forested, mountainous watersheds that provide much of the water supply in western North America. To address this knowledge gap, we configured SWAT to simulate the streamflow of Tenderfoot Creek (TCSWAT). Located in central Montana, TCSWAT represents a high‐elevation watershed with ～85% coniferous forest cover where more than 70% of the annual precipitation falls as snow, and runoff comes primarily from spring snowmelt. Model calibration using four years of measured daily streamflow, temperature, and precipitation data resulted in a relative error (RE) of 2% for annual water yield estimates, and mean paired deviations (Dv) of 36 and 31% and Nash‐Sutcliffe (NS) efficiencies of 0.90 and 0.86 for monthly and daily streamflow, respectively. Model validation was conducted using an additional four years of data and the performance was similar to the calibration period, with RE of 4% for annual water yields, Dv of 43% and 32%, and NS efficiencies of 0.90 and 0.76 for monthly and daily streamflow, respectively. An objective, regression‐based model invalidation procedure also indicated that the model was validated for the overall simulation period. Seasonally, SWAT performed well during the spring and early summer snowmelt runoff period, but was a poor predictor of late summer and winter base flow. The calibrated model was most sensitive to snowmelt parameters, followed in decreasing order of influence by the surface runoff lag, ground water, soil, and SCS Curve Number parameter sets. Model sensitivity to the surface runoff lag parameter reflected the influence of frozen soils on runoff processes. Results indicated that SWAT can provide reasonable predictions of annual, monthly, and daily streamflow from forested montane watersheds, but further model refinements could improve representation of snowmelt runoff processes and performance during the base flow period in this environment.     

Estimating Evapotranspiration and Seepage for a Sinkhole Wetland From Diurnal Surface‐Water Cycles1

Abstract: This study used measured diurnal surface‐water cycles to estimate daily evapotranspiration (ET) and seepage for a seasonally flooded sinkhole wetland. Diurnal surface‐water cycles were classified into five categories based on the relationship between the surface‐water body and the surrounding ground‐water system (i.e., recharge/discharge). Only one class of diurnal cycles was found to be suitable for application of this method. This subset of diurnal cycles was used to estimate ET and seepage and the relative importance of each transfer process to the overall water budget. The method has limited utility for wetlands with erratic hydrologic regimes (e.g., wetlands in urban environments). This is due to violation of the critical assumption that the inflow/outflow rate remains constant throughout the day. For application to surface‐water systems, the method is typically applied with an assumed specific yield of 1.0. This assumption was found to be invalid for application to surface‐water systems with a noncylindrical pond geometry. An overestimation of ET by as much as 60% was found to occur under conditions of low pond stage and high water loss. The results demonstrate the high ET rates that can occur in isolated wetlands due to contrasting roughness and moisture conditions (oasis and clothesline effects). Estimated ET rates ranged from 4.1 to 18.7 mm/day during the growing season. Despite these large ET rates, seepage (recharge) was found to be the dominant water loss mechanism for the wetland.     

WATER BUDGETS FOR SMALL DIKED MARSHES1

ABSTRACT: This paper reports an analysis of the water budgets of 10 small (5–6 ha) diked areas (cells) within the Delta Marsh in southcentral Manitoba, Canada. The important terms in the water budget equation in this study were precipitation (P), water pumped in (SWI), evapotranspiration (ET), seepage in (GWI) and out (GWO), and change in storage (ΔS). P, SWI, and S were measured directly, and the sum of ET and GWO determined by difference. Estimating ET as 0.7 pan evaporation gave a seepage loss of 2.9 mm/day from the most intensively studied cell. Other methods of estimating ET produced estimates of GWO ranging from 2.4 to 3.8 mm/day. Water budgets for less intensively studied cells indicated seepage loss increased as perimeter available for seepage increased, but not proportionately. Efforts to measure seepage directly or estimate it from measured hydraulic gradients and hydraulic conductivity produced estimates much lower than the estimates from the water budget equation. Hydraulic conductivities were very heterogeneous, reflecting the sorting of water deposited sediments. Comparison of the hydraulic conductivities with seepage estimates from the water budget strongly suggests water movement downward as well as laterally from these diked areas.