EVAPOTRANSPIRATION CONCEPTUALIZATION IN THE HSPF‐MODFLOW INTEGRATED MODELS1

In 1988, the Florida Institute of Phosphate Research (FIPR) funded project to develop an advanced hydrologic model for shallow water table systems. The FIPR hydrologic model (FHM) was developed to provide an improved predictive capability of the interactions of surface water and ground water using its component models, HSPF and MODFLOW. The Integrated Surface and Ground Water (ISGW) model was developed from an early version of FHM and the two models were developed relatively independently in the late 1990s. Hydrologic processes including precipitation, interception, evapotranspiration, runoff, recharge, streamflow, and base flow are explicitly accounted for in both models. Considerable review of FHM and ISGW and their applications occurred through a series of projects. One model evolved, known as the Integrated Hydrological Model IHM. This model more appropriately describes hydrologic processes, including evapotranspiration fluxes within small distributed land‐based discretization. There is a significant departure of many IHM algorithms from FHM and ISGW, especially for soil water and evapotranspiration (ET). In this paper, the ET concepts in FHM, ISGW, and IHM will be presented. The paper also identifies the advantages and data costs of the improved methods. In FHM and IHM, ground water ET algorithms of the MODFLOW ET package replace those of HSPF (ISGW used a different model for ground water ET). However, IHM builds on an improved understanding and characterization of ET partitioning between surface storages, vadose zone storage, and saturated ground water storage. The IHM considers evaporative flux from surface sources, proximity of the water table to land surface, relative moisture condition of the unsaturated zone, thickness of the capillary zone, thickness of the root zone, and relative plant cover density. The improvements provide a smooth transition to satisfy ET demand between the vadose zone and deeper saturated ground water. While the IHM approach provides a more sound representation of the actual soil profile than FHM, and has shown promise at reproducing soil moisture and water table fluctuations as well as field measured ET rates, more rigorous testing is necessary to understand the robustness and/or limitations of this methodology.     

MODELING THE HYDROLOGIC IMPACTS OF FOREST HARVESTING ON FLORIDA FLATWOODS1

ABSTRACT: The great temporal and spatial variability of pine flat-woods hydrology suggests traditional short-term field methods may not be effective in evaluating the hydrologic effects of forest management. The FLATWOODS model was developed, calibrated and validated specifically for the cypress wetland-pine upland landscape. The model was applied to two typical flatwoods sites in north central Florida. Three harvesting treatments (Wetland Harvesting, Wetland + Upland Harvesting, and Control) under three typical climatic conditions (dry, wet, and normal precipitation years) were simulated to study the potential first-year effects of common forest harvesting activities on flatwoods. Long-term (15 years) simulation was conducted to evaluate the hydrologic impacts at different stages of stand rotation. This simulation study concludes that forest harvesting has substantial effects on hydrology during dry periods and clear cutting of both wetlands and uplands has greater influence on the water regimes than partial harvesting. Compared to hilly regions, forest harvesting in the Florida coastal plains has less impact on water yield.     

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.     

EVAPOTRANSPIRATION FROM FLORIDA PONDCYPRESS SWAMPS1

ABSTRACT: We measured diurnal changes in water levels in three swamps dominated by pondcypress trees (Taxodium distichum var. nuans) in central Florida for four years in order to obtain additional documentation of relatively low evapotranspiration (ET) rates. Two of these swamps were monitored for another three years after one of them was clearcut. Estimated annual ET from undisturbed cypress swamps varied from 38 cm/yr to 86 cm/yr, averaging 60 cm (not including interception). Faster ET rates may have been related to faster pondcypress growth rates, a greater proportion of hardwoods in the canopy, and clearcutting in the surrounding pine plantation. The average ET rate was considerably lower than ET rates that have been estimated for north Florida pine plantations. However, incorporating estimates of interception indicates that overall ET rates in pondcypress swamps may be only slightly lower than ET from pine plantations. ET decreased only 5 percent in one swamp after it was clearcut, indicating that this management practice is not likely to affect regional water balances.     

SIMULATION OF INTEGRATED SURFACE WATER AND GROUND WATER SYSTEMS - MODEL FORMULATION1

ABSTRACT: The unique characteristics of the hydrogeologic system of south Florida (flat topography, sandy soils, high water table, and highly developed canal system) cause significant interactions between ground water and surface water systems. Interaction processes involve infiltration, evapotranspiration (ET), runoff, and exchange of flow (seepage) between streams and aquifers. These interaction processes cannot be accurately simulated by either a surface water model or a ground water model alone because surface water models generally oversimplify ground water movement and ground water models generally oversimplify surface water movement. Estimates of the many components of flow between surface water and ground water (such as recharge and ET) made by the two types of models are often inconsistent. The inconsistencies are the result of differences in the calibration components and the model structures, and can affect the confidence level of the model application. In order to improve model results, a framework for developing a model which integrates a surface water model and a ground water model is presented. Dade County, Florida, is used as an example in developing the concepts of the integrated model. The conceptual model is based on the need to evaluate water supply management options involving the conjunctive use of surface water and groundwater, as well as the evaluation of the impacts of proposed wellfields. The mathematical structure of the integrated model is based on the South Florida Water Management Model (SFWMM) (MacVicar et al., 1984) and A Modular Three-Dimensional Finite-Difference Groundwater Flow Model (MODFLOW) (McDonald and Harbaugh, 1988).     

EVAPOTRANSPIRATION AND SOIL MOISTURE DYNAMICS ON A SEMIARID PONDEROSA PINE HILLSLOPE1

ABSTRACT: Detailed measurements of soil moisture and ET in semiarid forest environments have not been widely reported in the literature. In this study, soil moisture and water balance components were measured over a four‐year period on a semiarid ponderosa pine hillslope, with evapotranspiration (ET) determined as the residual of measured precipitation, runoff, and change in soil moisture storage. ET accounts for approximately 95 percent of the water budget and has a distinctly bimodal annual pattern, with peaks occurring after spring snowmelt and during the late summer monsoon season, periods that coincide with high soil moisture. Weekly growing season ET rates determined by the hillslope water balance are found to be invariably below calculated potential rates. Normalized ET rates are linearly correlated (r²= 0.62) with soil moisture; therefore, a simple linear relation is proposed. Growing season soil moisture dynamics were modeled based on this relation. Results are in fair agreement (r²= 0.63) with the observed soil moisture data over the four growing seasons; however, for two dry summers with little surface runoff, much better results (r² > 0.90) were obtained.     

SIMULATING THE EFFECTS OF REDUCED PRECIPITATION ON GROUND WATER AND STREAMFLOW IN THE NEBRASKA SAND HILLS1

ABSTRACT: The Nebraska Sand Hills have a unique hydrologic system with very little runoff and thick aquifers that constantly supply water to rivers, lakes, and wetlands. A ground water flow model was developed to determine the interactions between ground water and streamflow and to simulate the changes in ground water systems by reduced precipitation. The numerical modeling method includes a water balance model for the vadose zone and MOD‐FLOW for the saturated zone. The modeling results indicated that, between 1979 and 1990, 13 percent of the annual precipitation recharged to the aquifer and annual ground water loss by evapotranspiration (ET) was only about one‐fourth of this recharge. Ground water discharge to rivers accounts for about 96 percent of the streamflow in the Dismal and Middle Loup rivers. When precipitation decreased by half the average amount of the 1979 to 1990 period, the average decline of water table over the study area was 0.89 m, and the streamflow was about 87 percent of the present rate. This decline of the water table results in significant reductions in ET directly from ground water and so a significant portion of the streamflow is maintained by capture of the salvaged ET.     

Sensitivity and Uncertainty of Ground‐Water Discharge Estimates for Semiarid Shrublands1

Abstract: We proposed a step‐by‐step approach to quantify the sensitivity of ground‐water discharge by evapotranspiration (ET) to three categories of independent input variables. To illustrate the approach, we adopt a basic ground‐water discharge estimation model, in which the volume of ground water lost to ET was computed as the product of the ground‐water discharge rate and the associated area. The ground‐water discharge rate was assumed to equal the ET rate minus local precipitation. The objective of this study is to outline a step‐by‐step procedure to quantify the contributions from individual independent variable uncertainties to the uncertainty of total ground‐water discharge estimates; the independent variables include ET rates of individual ET units, areas associated with the ET units, and precipitation in each subbasin. The specific goal is to guide future characterization efforts by better targeting data collection for those variables most responsible for uncertainty in ground‐water discharge estimates. The influential independent variables to be included in the sensitivity analysis are first selected based on the physical characteristics and model structure. Both regression coefficients and standardized regression coefficients for the selected independent variables are calculated using the results from sampling‐based Monte Carlo simulations. Results illustrate that, while as many as 630 independent variables potentially contribute to the calculation of the total annual ground‐water discharge for the case study area, a selection of seven independent variables could be used to develop an accurate regression model, accounting for more than 96% of the total variance in ground‐water discharge. Results indicate that the variability of ET rate for moderately dense desert shrubland contributes to about 75% of the variance in the total ground‐water discharge estimates. These results point to a need to better quantify ET rates for moderately dense shrubland to reduce overall uncertainty in estimates of ground‐water discharge. While the approach proposed here uses a basic ground‐water discharge model taken from an earlier study, the procedure of quantifying uncertainty and sensitivity can be generalized to handle other types of environmental models involving large numbers of independent variables.     

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.     

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.     

APPLICATION OF A HYDROLOGIC MODEL FOR LAND USE PLANNING IN FLORIDA1

ABSTRACT: An environmental simulation model of the Upper St. Johns River Basin, Florida, has been developed in order to predict hydrologic responses under proposed management plans. Land use projections for each of 19 hydrologic planning units are provided by a linear programming analysis of agricultural activities. Inputs to the model include rainfall, runoff, evapotranspiration (ET), aquifer properties, topography, soil types, and vegetative patterns. A water balance is developed in the uplands based on infiltration, ET, surface runoff, and groundwater flow. Valley continuity is based on stage-volume relationship for inflows and outflows and a variable roughness coefficient dependent on vegetative patterns. Land use changes form the basis for predicting hydroperiod variation under alternative management schemes. Plans are ranked according to two criteria, deviation from a natural hydroperiod and flood or drought control provided. Results indicate that (1) a single reservoir without irrigation and (2) floodplain preservation plans are superior to (3) multiple reservoir with irrigation and (4) uncontrolled floodplain plans with regard to both criteria.     

MODELING EFFECTS OF BRUSH MANAGEMENT ON THE RANGELAND WATER BUDGET: EDWARDS PLATEAU,TEXAS1

ABSTRACT: The Soil and Water Assessment Tool (SWAT) was used to evaluate the influence of woody plants on water budgets of semi‐arid rangelands in karst terrain. The model was configured for the hydrologic evaluation of the North Fork of the Upper Guadalupe River watershed and was calibrated and verified using measured flow data. Nash and Sutcliffe fit efficiencies for daily and monthly verification periods were 0.09 and 0.50, respectively. Streamflow, baseflow, and evapotranspiration (ET) rates were comparable to published field data. Simulated deep recharge was considerably higher than the published values for the Edwards Plateau. The model was then used to simulate conditions with existing brush cover and four different brush removal scenarios. Scenarios were created to represent existing brush and the removal of brush from only locations that were either covered by heavy brush, were on a moderate slope, or were in shallow soils. Resulting data was compared to previous studies of both field experiments and model simulations. Maximum brush removal resulted in a reduction in ET equal to 31.94 mm/yr depth over the entire basin, or 46.62 mm/yr depth over the treated area. Removal of heavy brush cover resulted in the greatest changes in evapotranspiration, surface runoff, base‐flow, and deep recharge. Slope was found to have the greatest effect on lateral subsurface flow.     

EMERGENCY WATER SUPPLIES FROM GROUND WATER IN HUMID REGIONS1

ABSTRACT: This paper focuses on the development and testing of a mathematical model of an emergency ground water supply operated principally during periods of low streamflow. The process of ground water withdrawal and recharge is simulated taking account of streamflow, water demand, evapotranspiration, natural and artificial recharge and increased evapotranspiration due to artificial recharge, ground water pumpage, and streamflow contribution to pumped water. The model determines whether natural recharge is possible in less time than the return period of drought and also whether artificial recharge is needed. By simulating operation over a long period of time, the model can examine different droughts of short and long duration and can test the operating rules for ground water storage development in an area. Submodels analyze the components of the operating process including ground water flow into the stream, seepage losses, stream portion of well discharge due to induced infiltration and recharge from rainfall or water spreading. The model has been tested for areas in the humid northeastern United States.     

Estimating Forest Ecosystem Evapotranspiration at Multiple Temporal Scales With a Dimension Analysis Approach1

Abstract:  It is critical that evapotranspiration (ET) be quantified accurately so that scientists can evaluate the effects of land management and global change on water availability, streamflow, nutrient and sediment loading, and ecosystem productivity in watersheds. The objective of this study was to derive a new semi‐empirical ET modeled using a dimension analysis method that could be used to estimate forest ET effectively at multiple temporal scales. The model developed describes ET as a function of water availability for evaporation and transpiration, potential ET demand, air humidity, and land surface characteristics. The model was tested with long‐term hydrometeorological data from five research sites with distinct forest hydrology in the United States and China. Averaged simulation error for daily ET was within 0.5 mm/day. The annual ET at each of the five study sites were within 7% of measured values. Results suggest that the model can accurately capture the temporal dynamics of ET in forest ecosystems at daily, monthly, and annual scales. The model is climate‐driven and is sensitive to topography and vegetation characteristics and thus has potential to be used to examine the compounding hydrologic responses to land cover and climate changes at multiple temporal scales.     

ANNUAL EVAPOTRANSPIRATION OF NATWE VEGETATION IN A MEDITERRANEAN-TYPE CLIMATE1

ABSTRACT: Annual evapotranspiration from a watershed (ETws) is a function of annual precipitation (P) and fraction of the watershed covered by shrubs and trees (C). Other characteristics are not significant in explaining variance of ETws. A rational equation, ETws = (1-C) ETg + CETst, wherein ETg is the ET of herbaceous cover and ETst is the ET of shrubs and trees, is proposed. The equation has been calibrated for ET and P in inches for the watershed of Lake Cachuma on the Santa Ynez River. This equation, ETmax = 2.14(1-C)P^0.647+ 4.53 C^1.76 P^0.68, is recommended for estimating maximum annual ET demand for conceptual models. Where C is not known, the upper limit of ET = f (P) may be approximated by use of 0.65 for C. The equation has been derived for large unmanaged watersheds. Applicability for evaluation of contemporary multiple purpose vegetation management should be determined by studies of the hydrology of small openings in shrub and tree cover.     

EFFECTS OF CONTROLLED DRAINAGE ON STORM EVENT HYDROLOGY IN A LOBLOLLY PINE PLANTATION1

ABSTRACT: A paired watershed approach was utilized to study the effects of three water management regimes on storm event hydrology in three experimental watersheds in a drained loblolly pine (Pinus taeda L.) plantation in eastern North Carolina. The regimes were: (1) conventional drainage, (2) controlled drainage (CD) to reduce outflows during spring fish recruitment, and (3) controlled drainage to reduce outflows and conserve water during the growing season. Data from two pit‐treatment years and three years of CD treatment with raised weirs at the watershed outlet are presented. CD treatment resulted in rises in water table elevations during the summer. But the rises were small and short‐lived due to increased evapotranspiration (ET) rates as compared to the spring treatment with lower ET demands. CD treatment had no effect on water tables deeper than 1.3 m. CD treatments, however, significantly (α= 0.05) reduced the stoning outflows for all events, and peak outflow rates for most of the events depending upon the outlet weir level. In some events, flows did not occur at all in watersheds with CD. When event outflows occurred, duration of the event was sharply reduced because of reduced effective ditch depth. Water table depth at the start of an event influenced the effect of CD treatment on storm event hydrology.     

DEVELOPMENT AND APPLICATION OF A SPATIAL HYDROLOGY MODEL OF OKEFENOKEE SWAMP,GEORGIA1

ABSTRACT: :The model described herein was used to assess effects of the Suwannee River sill (a low earthen dam constructed to impound the Suwannee River within the Okefenokee National Wildlife Refuge to eliminate wildfires) on the hydrologic environment of Okefenokee Swamp, Georgia. Developed with Arc/Info Macro Language routines in the GRID environment, the model distributes water in the swamp landscape using precipitation, inflow, evapotranspiration, outflow, and standing water. Water movement direction and rate are determined by the neighborhood topographic gradient, determined using survey grade Global Positioning Systems technology. Model data include flow rates from USGS monitored gauges, precipitation volumes and water levels measured within the swamp, and estimated evapotranspiration volumes spatially modified by vegetation type. Model output in semi‐monthly time steps includes water depth, water surface elevation above mean sea level, and movement direction and volume. Model simulations indicate the sill impoundment affects 18 percent of the swamp during high water conditions when wildfires are scarce and has minimal spatial effect (increasing hydroperiods in less than 5 percent of the swamp) during low water and drought conditions when fire occurrence is high but precipitation and inflow volumes are limited.     

ESTIMATION OF REGIONAL EVAPOTRANSPIRATION FROM ARID AND SEMI-ARID SURFACES1

ABSTRACT: Evapotranspiration (ET) from oasis and Gobi surfaces in the Heihe River region of China was estimated by Kotoda (1986) and Advection-Aridity (Brutsaert and Stricker, 1979) models. The ET estimates were compared with eddy-correlation ET estimates. The ET estimated by both models are in good agreement with ET obtained by means of eddy-correlation method for the oasis surface while underestimating ET as compared to the eddy-correlation method. For the Gobi surface, the models yielded obviously overestimates of evapotranspiration. In order to estimate evapotranspiration from arid surfaces, the Kotoda model was modified by introducing the surface moisture availability parameter a from Barton (1979). The modified Kotoda model yielded ET estimates that were very close to that from the eddy-correlation method for the Gobi surface. The modified Kotoda model was used to estimate evapotranspiration from the Heihe River watershed, an area with complicated topography and land use, and the results compared with those from a water balance method. A sensitivity analysis of the modified model was performed. The results show that the modified Kotoda model can reflect the relationship between the actual evapotranspiration and the main controlling factors on it for both wet and arid surfaces reasonably. From this study, it can be concluded that the modified Kotoda model is applicable fro the estimation of regional evapotranspiration from areas with complicated topography and land use.     

REGIONAL HYDROLOGIC RESPONSE OF LOBLOLLY PINE TO AIR TEMPERATURE AND PRECIPITATION CHANGES1

ABSTRACT: Large deviations in average annual air temperatures and total annual precipitation were observed across the southern United States during the last 50 years, and these fluctuations could become even larger during the next century. We used PnET-IIS, a monthly time-step forest process model that uses soil, vegetation, and climate inputs to assess the influence of changing climate on southern U.S. pine forest water use. After model predictions of historic drainage were validated, the potential influences of climate change on loblolly pine forest water use was assessed across the region using historic (1951 to 1984) monthly precipitation and air temperature which were modified by two general circulation models (GCMs). The GCMs predicted a 3.2°C to 7.2°C increase in average monthly air temperature, a -24 percent to + 31 percent change in monthly precipitation and a -1 percent to + 3 percent change in annual precipitation. As a comparison to the GCMs, a minimum climate change scenario using a constant 2°C increase in monthly air temperature and a 20 percent increase in monthly precipitation was run in conjunction with historic climate data. Predicted changes in forest water drainage were highly dependent on the GCM used. PnET-IIS predicted that along the northern range of loblolly pine, water yield would decrease with increasing leaf area, total evapotranspiration and soil water stress. However, across most of the southern U.S., PnET-IIS predicted decreased leaf area, total evapotranspiration, and soil water stress with an associated increase in water yield. Depending on the GCM and geographic location, predicted leaf area decreased to a point which would no longer sustain loblolly pine forests, and thus indicated a decrease in the southern most range of the species within the region. These results should be evaluated in relation to other changing environmental factors (i.e., CO₂ and O₃) which are not present in the current model.     

SELECTION OF APPROPRIATE EVAPORATION ESTIMATION TECHNIQUE FOR CONTINUOUS MODELING1

ABSTRACT: The importance of evaporation in hydrologic modeling has been investigated by analysis of water budget at various scales and application of a water management model at plot scale. The data at all scales indicate that evaporation constitutes a major hydrological output. Accurate determination of watershed evapotranspiration depends upon appropriate accounting for the various components of evapotranspiration and their areal and temporal variability. An application of a water management model has revealed that the ranking of the sensitivity of input parameters depends upon the method of determination of evapotranspiration.