MODELING THE FOREST HYDROLOGY OF WETLAND-UPLAND ECOSYSTEMS IN FLORIDA1

ABSTRACT: Few hydrological models are applicable to pine flat-woods which are a mosaic of pine plantations and cypress swamps. Unique features of this system include ephemeral sheet flow, shallow dynamic ground water table, high rainfall and evapotranspiration, and high infiltration rates. A FLATWOODS model has been developed specifically for the cypress wetland-pine upland landscape by integrating a 2-D ground water model, a Variable-Source-Area (VAS)-based surface flow model, an evapotranspiration (ET) model, and an unsaturated water flow model. The FLATWOODS model utilizes a distributed approach by dividing the entire simulation domain into regular cells. It has the capability to continuously simulate the daily values of ground water table depth, ET, and soil moisture content distributions in a watershed. The model has been calibrated and validated with a 15-year runoff and a four-year ground water table data set from two different pine flat woods research watersheds in northern Florida. This model may be used for predicting hydrologic impacts of different forest management practices in the coastal regions.     

IMPACTS OF SILVICULTURE ON FLATWOODS RUNOFF,WATER QUALITY,AND NUTRIENT BUDGETS1

ABSTRACT: Three forest watersheds were isolated by roads in poorly drained flatwoods of Florida. After 12 months of baseline calibration the forest in one watershed was harvested and regenerated with minimum disturbance, in the second watershed with maximum disturbance from common practices, and in the third watershed left intact as a control. Water yields from the maximum treatments increased a significant 250 percent while that from the minimum treatments increased 117 percent as compared to the control. Weed vegetation remaining after the minimum treatment continued significant water use. The water yield increases lasted only for one year. Water quality was reduced by both treatments with the most effect immediately after the maximum disturbance. Absolute levels of suspended sediments, potassium, and calcium remained relatively low. The maximum treatment caused significant changes in net cation balances only for one year. The information shows relative little effect of silvicultural practices in flatwoods on water quality as compared to data from upland forests. Water yield increases may be manipulated by the degree of harvest and weed control practices.     

EVALUATION OF HYDROLOGIC MODELING TECHNIQUES FOR FOREST LAND MANAGEMENT PLANNING1

ABSTRACT: A growing concern for environmental quality paralleled with increasing demands on our forest resources has prompted the Washington State Department of Natural Resources to evaluate simulation modeling as a technique for analyzing management decisions in terms of their environmental effects. The evaluation focused on a system of integrated models developed at the University of Washington which simulate processes and activities within the forest ecosystem. A major part of the system is a hydrologic model which predicts changes in discharge, stream temperature, and concentrations of suspended sediment and dissolved oxygen based on information generated by other models representing intensive management practices. The evaluation consisted of applying the system to a 72,000 acre tract of forest land, validating the models with two years of discharge and water quality data from a 93,000 acre watershed, and determining the pertinence of hydrologic modeling for management purposes. Results show several potential uses of hydrologic modeling for forest management planning, especially for analyzing the effects of timber harvesting strategies on water quality.     

DETERMINISTIC HYDROLOGIC MODELING OF GRAZING SYSTEM IMPACTS ON INFILTRATION RATES1

ABSTRACT: Techniques for predicting the hydrologic effects of grazing schemes have heretofore been unavailable. The available literature on grazing intensity influences on infiltration rates is used as a basis for a model of infiltration behavior in response to grazing systems. Background, development, cautions, and an example are given.     

RASTER-BASED HYDROLOGIC MODELING OF SPATIALLY-VARIED SURFACE RUNOFF1

ABSTRACT: The proliferation of watershed databases in raster Geographic Information System (GIS) format and the availability of radar-estimated rainfall data foster rapid developments in raster-based surface runoff simulations. The two-dimensional physically-based rainfall-runoff model CASC2D simulates spatially-varied surface runoff while fully utilizing raster GIS and radar-rainfall data. The model uses the Green and Ampt infiltration method, and the diffusive wave formulation for overland and channel flow routing enables overbank flow storage and routing. CASC2D offers unique color capabilities to display the spatio-temporal variability of rainfall, cumulative infiltrated depth, and surface water depth as thunderstorms unfold. The model has been calibrated and independently verified to provide accurate simulations of catchment response to moving rainstorms on watersheds with spatially-varied infiltration. The model can accurately simulate surface runoff from flashfloods caused by intense thunderstorms moving across partial areas of a watershed.     

CLIMATE CHANGE IMPACTS ON THE HYDROLOGY AND PRODUCTIVITY OF A PINE PLANTATION1

ABSTRACT: There are increasing concerns in the forestry community about global climate change and variability associated with elevated atmospheric CO₂. Changes in precipitation and increases in air temperature could impose additional stress on forests during the next century. For a study site in Carteret County, North Carolina, the General Circulation Model, HADCM2, predicts that by the year 2099, maximum air temperature will increase 1.6 to 1.9°C, minimum temperature will increase 2.5 to 2.8°C, and precipitation will increase 0 to 10 percent compared to the mid‐1990s. These changes vary from season to season. We utilized a forest ecosystem process model, PnET‐II, for studying the potential effects of climate change on drainage outflow, evapotranspiration, leaf area index (LAI) and forest Net Primary Productivity (NPP). This model was first validated with long term drainage and LAI data collected at a 25‐ha mature loblolly pine (Pinus taeda L.) experimental watershed located in the North Carolina lower coastal plain. The site is flat with poorly drained soils and high groundwater table. Therefore, a high field capacity of 20 cm was used in the simulation to account for the topographic effects. This modeling study suggested that future climate change would cause a significant increase of drainage (6 percent) and forest productivity (2.5 percent). Future studies should consider the biological feedback (i.e., stomata conductance and water use efficiency) to air temperature change.     

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.     

SPACE-TIME MODELING OF VECTOR HYDROLOGIC SEQUENCES1

Stochastic modeling of vector hydrologic sequences is examined with a general class of space-time autoregressive integrated moving average (STARIMA) models. The models describe spatial and temporal autocorrelatjon, through dependent variables lagged both in space and time. The model structures incorporate a hierarchical ordering scheme to map the vector of observations into a network configuration. The neighboring structure used introduces a physical/geographical hierarchy to enable the model identification procedures to assist in determining appropriate correlative relationships. The three-stage iterative space-time model building procedure is illustrated using average monthly streamfiow data for a four-station network of the Southeastern Hydropower System.     

HYDROLOGIC EFFECTS OF TWO METHODS OF HARVESTING MATURE SOUTHERN PINE1

ABSTRACT: Hydrologic responses to logging with skidders and responses to logging with a cable yarder are compared. After a 23-year calibration with an undisturbed control catchment, mixed stands of shortleaf pine (Pinus echinata Mill.) and hardwoods were clearfelled on two small catchments in the hilly Coastal Plain of north Mississippi and observed for five years. Runoff increased 370 mm (skidded) and 116 mm (yarded) during the first year with 1876 mm of rainfall, and 234 mm (skidded) and 228 mm (yarded) during the second year when 1388 mm of precipitation equaled the calibration mean. Sediment concentrations for the yarded catchment during the first two years averaged 641 and 1,629 mg L^?1, respectively, and yields were 6,502 and 12,086 kg ha^?1. Compared to calibration means of 74 mg L^?1 and 142 kg ha^?1, these extreme values can be attributed largely to transport of sediment stored in the channel and to erosion of subsurface flow paths, which was exacerbated by high flow volumes. During the first year, the concentration (231 mg L^?1) and yield (2,827 kg ha^?1) for the control catchment also exceeded the calibration means. However, concentrations (134 mg L^?1) and yields (1,806 kg ha^?1) for the skidded catchment were about 40 percent lower than for the control catchment during the first year, and were higher than those for the control only during the second year. Because deep percolation was limited and because rainfall was unusually high, increases in flows and sediment concentrations and yields probably approximate maximum responses to clearcut harvesting in the uplands of the southern Coastal Plain.     

POTENTIAL CLIMATE CHANGE IMPACTS ON MOUNTAIN WATERSHEDS IN THE PACIFIC NORTHWEST1

ABSTRACT: Global climate change due to the buildup of greenhouse gases in the atmosphere has serious potential impacts on water resources in the Pacific Northwest. Climate scenarios produced by general circulation models (GCMs) do not provide enough spatial specificity for studying water resources in mountain watersheds. This study uses dynamical downscaling with a regional climate model (RCM) driven by a GCM to simulate climate change scenarios. The RCM uses a subgrid parameterization of orographic precipitation and land surface cover to simulate surface climate at the spatial scale suitable for the representation of topographic effects over mountainous regions. Numerical experiments have been performed to simulate the present-day climatology and the climate conditions corresponding to a doubling of atmospheric CO₂ concentration. The RCM results indicate an average warming of about 2.5°C, and precipitation generally increases over the Pacific Northwest and decreases over California. These simulations were used to drive a distributed hydrology model of two snow dominated watersheds, the American River and Middle Fork Flathead, in the Pacific Northwest to obtain more detailed estimates of the sensitivity of water resources to climate change. Results show that as more precipitation falls as rain rather than snow in the warmer climate, there is a 60 percent reduction in snowpack and a significant shift in the seasonal pattern of streamflow in the American River. Much less drastic changes are found in the Middle Fork Flathead where snowpack is only reduced by 18 percent and the seasonal pattern of streamflow remains intact. This study shows that the impacts of climate change on water resources are highly region specific. Furthermore, under the specific climate change scenario, the impacts are largely driven by the warming trend rather than the precipitation trend, which is small.     

INTEGRATION OF FOREST ECOSYSTEM AND CLIMATIC MODELS WITH A HYDROLOGIC MODEL1

ABSTRACT: The effects of changes in the landscape and climate over geological time are plain to see in the present hydrological regime. More recent anthropogenic changes may also have effects on our way of life. A prerequisite to predicting such effects is that we understand the interactions between climate, landscape and the hydrological regime. A semi-distributed hydrological model (SLURP) has been developed which can be used to investigate, in a simple way, the links between landscape, climate and hydrology for watersheds of various sizes. As well as using data from the observed climate network, the model has been used with data from atmospheric models to investigate possible changes in hydrology. A critical input to such a model is knowledge of the links between landscape and climate. While direct anthropogenic effects such as changes in forested area may presently be included, the indirect effects of climate on landscape and vice versa are not yet modeled well enough to be explicitly included. The development of models describing climate-landscape relationships such as regeneration, development and breakup, water and carbon fluxes at species, ecosystem and biome level is a necessary step in understanding and predicting the effects of changes in climate on landscape and on water resources. Forest is the predominant land cover in Canada covering 453 Mha and productivity/succession models for major forest types should be included in an integrated climate-landscape-water simulation.     

MODELING HYDROLOGIC TIME SERIES FROM THE ARCTIC1

The general intervention model is applied to hydrologic and meteorologjc time series from the Canadian Arctic. The authors show how the model is able to account for environmental interventions, missing observations in the data, changes in data collection procedures, the effects of external inputs, as well as seasonality and autocorrelation. Methods for identifying transfer functions by making use of a physical understanding of the processes involved are demonstrated and sample applications of the general intervention model to Arctic data are shown.     

POTENTIAL IMPACTS OF CLIMATE CHANGE ON CALIFORNIA HYDROLOGY1

ABSTRACT: Previous reports based on climate change scenarios have suggested that California will be subjected to increased wintertime and decreased summertime streamflow. Due to the uncertainty of projections in future climate, a new range of potential climatological future temperature shifts and precipitation ratios is applied to the Sacramento Soil Moisture Accounting Model and Anderson Snow Model in order to determine hydrologic sensitivities. Two general circulation models (GCMs) were used in this analysis: one that is warm and wet (HadCM2 run 1) and one that is cool and dry (PCM run B06.06), relative to the GCM projections for California that were part of the Third Assessment Report of the Intergovernmental Panel on Climate Change. A set of specified incremental temperature shifts from 1.5°C to 5.0°C and precipitation ratios from 0.70 to 1.30 were also used as input to the snow and soil moisture accounting models, providing for additional scenarios (e.g., warm/dry, cool/wet). Hydrologic calculations were performed for a set of California river basins that extend from the coastal mountains and Sierra Nevada northern region to the southern Sierra Nevada region; these were applied to a water allocation analysis in a companion paper. Results indicate that for all snow‐producing cases, a larger proportion of the streamflow volume will occur earlier in the year. The amount and timing is dependent on the characteristics of each basin, particularly the elevation. Increased temperatures lead to a higher freezing line, therefore less snow accumulation and increased melting below the freezing height. The hydrologic response varies for each scenario, and the resulting solution set provides bounds to the range of possible change in streamflow, snowmelt, snow water equivalent, and the change in the magnitude of annual high flows. An important result that appears for all snowmelt driven runoff basins, is that late winter snow accumulation decreases by 50 percent toward the end of this century.     

SIMULATION OF FRESHWATER DISCHARGES FROM UNGAGED AREAS TO THE SEBASTIAN RIVER,FLORIDA1

ABSTRACT: Due to alterations in the natural drainage system over the past several decades and intensified agricultural practices, freshwater discharges to the Sebastian River, Florida, have increased substantially. As a result, salinity patterns in the Sebastian River and adjacent Indian River lagoon have been disrupted and the influx of nutrients has increased. Recently, the St. Johns River Water Management District has developed a 3‐D hydrodynamic and salinity model for the Sebastian River and adjacent Indian River to study the effects of freshwater inflows, and to set guidelines for management of future freshwater discharges. Freshwater inflows to the Sebastian River are part of the input data of the hydrodynamic model. Except for the downstream drainage areas, inflows are gaged, and the data were used for calibration of the hydrologic simulations. Collectively, the downstream ungaged areas constitute about 16 percent of the total drainage area. Because of the significant contribution to the total drainage area, reliable estimates of freshwater discharges from the ungaged areas to the Sebastian River are needed. This case study illustrates the development of a set of model parameters, reflecting the hydrologic and physiographic characteristics of the entire region. In this context region applies to the watersheds located in the coastal area along the Indian River from Titusville in the north to Vero Beach in the south. The parameter set was first tested on a number of gaged drainage basins in the region, and was then applied to the ungaged areas.     

SWMHMS - SMALL WATERSHED MONTHLY HYDROLOGIC MODELING SYSTEM1

ABSTRACT: SWMHMS is a conceptual computer modeling program developed to simulate monthly runoff from a small nonurban watershed. The input needed to run model simulations include daily precipitation, monthly data for evapotranspiration determination (average temperature, crop consumptive coefficients, and percent daylight hours), and six watershed parameter values. Evapotranspiration was calculated with the Blaney-Criddle equation while surface runoff was determined using the Soil Conservation Service curve number procedure. For watershed parameter evaluation, SWMHMS provides options for both optimization and sensitivity analysis. Observed runoff data are required along with the model input previously mentioned in order to conduct parameter optimization. SWMEIMS was tested with data from six watersheds located in different regions of the United States. Model accuracy was generally found to be very good except on watersheds having substantial snowfall accumulation. In having only six watershed parameters, SWMHMS is less complex to use than many other computer programs that calculate monthly runoff. Consequently, SWMHMS may find its greatest application as an educational tool for students learning principles of hydrologic modeling, such as parameter evaluation procedures and the impacts of input data uncertainty on model results.     

FOREST HARVESTING AND WATER: THE LAKE STATES EXPERIENCE1

ABSTRACT: The impact of forests on water has_- been a subject of argument for more than a century. It still is; and many studies conform that there is no single right answer in the debate. In the Lake States, clearcutting natural peatlands will not change annual stream-flow nor will it seriously impact water quality if logging is done on frozen soils. However, clearcutting will cause water tables to fluctuate more, ranging from 9 cm higher to 19 cm lower than in peatlands with mature forests. Clearcutting upland hardwoods or conifers will increase annual strearnflow by 9 to 20 cm (a 30- to 80-percent increase). Streamfiow returns to preharvest levels in 12 to 15 years. Annual peak flows are at least doubled and snowmelt flood-peak increases may persist for 15 years. Water quality is not widely impacted, but operating logging equipment in stream channels will cause channel clogging by filamentous algae and loss of fish habitat. Permanent changes from forest to agricultural and urban land use on two-thirds or more of a watershed will significantly increase the size of flood peaks in the 2- to 30-year return interval storm or snowmelt.     

HYDROLOGIC RESPONSE OF MASSACHUSETTS WATERSHEDS1

Hydrologic response, defined as the annual direct runoff divided by the annual precipitation, was computed for twenty-one watersheds in or near western Massachusetts, using a total of 232 years of hydrologic records. Variability of the results over the period of analysis was greater than is desirable to inspire confidence in the usefulness of the hydrologic response function; however, the results do suggest that the hydrologic response concept, with appropriate refinements, could be applied successfully to the problem of delineating hydrologic provinces and determination of drainage and storage in unregulated watersheds.     

HYDROLOGIC AND ECONOMIC IMPACTS OF DROUGHT UNDER ALTERNATWE POLICY RESPONSES1

ABSTRACT: A severe sustained drought in the Colorado River Basin would cause economic damages throughout the Basin. An integrated hydrologic-economic-institutional model introduced here shows that consumptive water users in headwaters states are particularly vulnerable to very large shortfalls and hence large damages because their rights are effectively junior to downstream users. Chronic shortfalls to consumptive users relying on diversions in excess of rights under the Colorado River Compact are also possible. Nonconsumptive water uses (for hydropower and recreation) are severely affected during the worst drought years as instream flows are reduced and reservoirs are depleted. Damages to these uses exceeds those to consumptive uses, with the value of lost hydropower production the single largest economic impact of a severe sustained drought. Modeling of alternative policy responses to drought suggests three general policy approaches with particular promise for reducing damages. Consumptive use damages can be reduced by over 90 percent through reallocation from low to high valued uses and through reservoir storage strategies which minimize evaporation losses. Reservoir management to preserve minimum power pool levels for hydropower production (and to maintain reservoir recreation) may reduce damages to these nonconsumptive uses by over 30 percent, but it may increase consumptive use shortfalls.     

AN APPLICATION OF THE ANALYTIC ELEMENT METHOD IN MODELING FLORIDA EVERGLADES HYDROLOGY1

ABSTRACT: The large volumes of ground water that are discharged from the Everglades toward the Miami metropolitan area have historically posed a significant environmental water supply problem. In order to analyze the effects of seepage barriers on these subsurface outflows, the analytic element modeling code GFLOW was used to construct a ground water flow model of a region that includes a portion of the Everglades along with adjacent developed areas. The hydrology of this region can be characterized by a highly transmissive surficial aquifer in hydraulic contact with wetlands and canals. Calibration of the model to both wet and dry season conditions yielded satisfactory results, and it was concluded that the analytic element method is a suitable technique for modeling ground water flow in the Everglades environment. Finally, the model was used to evaluate the potential effectiveness of a subsurface barrier approximately two miles long for increasing water levels within the adjacent fringes of the Everglades National Park. It was found that the barrier had a negligible effect on water levels due to both its relatively short length and the high transmissivity of the surficial aquifer.     

FOREST HYDROLOGY IN THE PACIFIC NORTHWEST: ADDITIONAL RESEARCH NEEDS1

ABSTRACT: While much is known about the hydrology of forested mountain catchments in the Pacific Northwest, important research questions remain. For example, the dynamics of storm precipitation amounts and the modeling of catchment outflows represent a continuing research need. Without an improved understanding of the spatial and temporal aspects of storm precipitation patterns, our ability to evaluate and improve physically-based hydrologic models is limited. From a practical perspective, tens of thousands of kilometers of access roads have been constructed across forested catchments of the Pacific Northwest. Yet, few forestry research programs focus on road drainage (e.g., ditches, culverts, fords). The few studies that address this issue indicate road drainage systems need to function effectively over a wide range of flow events and terrain conditions. In addition, historical forest practices associated with hillslopes and riparian systems have altered the character of many Pacific Northwest aquatic ecosystems. If restoration of these systems is to be effective, research efforts are needed to better understand the linkages between riparian forests, geomorphic processes, and hydrologic disturbance regimes.