MORPHOLOGIC RESPONSE OF SUBALPINE STREAMS TO TRANSBASIN FLOW DIVERSION1

ABSTRACT: Flow has been diverted from Rocky Mountain streams for many years with little documentation of the impacts on physical form and biological function of the losing stream system. This study addresses whether differences in form can be detected in subalpine step-pool, plane bed, or pool-riffle channels and linked to changes in flow regime from diversion. Total annual discharge was reduced between 20 and 60 percent and average annual peak flow was reduced up to 45 percent in the subalpine systems assessed; channels were diverted between 20 and 100 years. Expected impacts include reduced conveyance and changes in vegetation growth patterns because formerly active surfaces are colonized by riparian species, effectuating shrinking channel capacity. In this study, reduced channel width is used as an indicator of morphologic response. Observed reductions in width, ranging from 35 to 50 percent at some sites, resulted not only from vegetation invasion of stable surfaces but also from the development of an inset beside former cut banks. This observation, however, was restricted to wider pool-riffle channels with gravel bars. Outside of these areas, morphologic changes were either subtle or absent. The absence of widespread response is attributed primarily to periodic “flooding” of the diverted channel. During wetter-than-average years when excess water is available, minimal flow is diverted and the hydrograph resembles a free-flowing regime. The release of high flow to the natural channel potentially offsets changes in form incurred during “dry” periods. The stable nature and structure of subalpine channels also contributes to the absence of reduced capacity.     

URBANIZATION OF AQUATIC SYSTEMS: DEGRADATION THRESHOLDS,STORMWATER DETECTION,AND THE LIMITS OF MITIGATION1

ABSTRACT: Urbanization of a watershed degrades both the form and the function of the downstream aquatic system, causing changes that can occur rapidly and are very difficult to avoid or correct. A variety of physical data from lowland streams in western Washington displays the onset of readily observable aquatic-system degradation at a remarkably consistent level of development, typically about ten percent effective impervious area in a watershed. Even lower levels of urban development cause significant degradation in sensitive water bodies and a reduced, but less well quantified, level of function throughout the system as a whole. Unfortunately, established methods of mitigating the downstream impacts of urban development may have only limited effectiveness. Using continuous hydrologic modeling we have evaluated detention ponds designed by conventional event methodologies, and our findings demonstrate serious deficiencies in actual pond performance when compared to their design goals. Even with best efforts at mitigation, the sheer magnitude of development activities falling below a level of regulatory concern suggests that increased resource loss will invariably accompany development of a watershed. Without a better understanding of the critical processes that lead to degradation, some downstream aquatic-system damage is probably inevitable without limiting the extent of watershed development itself.     

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.     

USING SENSITIVITY ANALYSIS TO ASSIST PARAMETER ZONATION IN GROUND WATER FLOW MODEL1

ABSTRACT: For numerical modeling of ground water movement in a real aquifer system, the aquifer is usually divided into hydrogeologically defined zones, each with its own parameter values. The responses of the system, such as head or drawdown, are often available only in some of the zones. The estimated parameters of all the zones are based on the measured response in these limited zones. However, the estimates for some of the zones may be very uncertain, and these zones are therefore not justified by the data. In this paper, an approach is presented to understand which zone may produce uncertain parameter values and should be lumped with its neighbor. This approach is demonstrated using a regional numerical model for pumping test analysis in the Nottinghamshire aquifer, UK. A step-by-step process is used in identifying the aquifer zones and estimating their parameters based on the principle of using the smallest possible numbers of zones and parameters for adequate representation of the drawdown response. After the parameters of each zone are estimated, the sensitivity features of these parameters are examined. The results show that the parameters in one zone can be estimated properly by the drawdown in another zone only when there is significant sensitivity. For transmissivity, sensitivity between zones occurs when there is significant flow between them. For storativity, sufficient sensitivity can occur without large flows between the zones, provided that one zone causes significant drawdown in the other. This idea can be extended to the flow model for a large aquifer system. If the aquifer is divided in such a way that aquifer responses are not sensitive to the parameters in some of the zones, the parameters in those zones cannot be estimated properly and should be lumped into their neighboring zones. In this way, a simple but more reasonable model can be built.     

A COMPARISON OF GEOSTATISTICAL METHODOLOGIES USED TO ESTIMATE SNOW WATER EQUIVALENT1

ABSTRACT: The need to monitor and forecast water resources accurately, particularly in the western United States, is becoming increasingly critical as the demand for water continues to escalate. Consequently, the National Weather Service (NWS) has developed a geostatistical model that is used to obtain areal estimates of snow water equivalent (the thtal water content in all phases of the snowpack), a major source of water in the West. The areal snow water equivalent estimates are used to update the hydrologic simulation models maintained by the NWS and designed to produce extended streamflow forecasts for river systems throughout the United States. An alternative geostatistical technique has been proposed to estimate snow water equivalent. In this research, we describe the two methodologies and compare the accuracy of the estimates produced by each technique. We illustrate their application and compare their estimation accuracy using snow data collected in the North Fork Clearwater River basin in Idaho.     

DRAINAGE GENERATION AND WATER USE IN THE EVERGLADES AGRICULTURAL AREA BASIN1

ABSTRACT: The Everglades Agricultural Area (EAA) covers 2,850 km² in area and is characterized by high water table and organic soil. The area is actively irrigated and drained as a function of weather conditions and crop status. Anthropogenic activities in the basin have resulted in nutrient-enriched drainage water that is discharged to Lake Okeechobee and the Everglades ecosystem. Water quantity and quality issues of the basin have become of increasing interest at local, state, and federal levels, so legislative and regulatory measures have been taken to improve water quality in discharges from the basin. In this study, simulation of hydrologic conditions and soil moisture were conducted using 100 years of daily synthetic rainfall data. From the simulations, the statistical distribution of half-month drainage discharge and supplemental water use in the basin was developed. The mean annual drainage/runoff was 49 cm, the mean supplemental water was 30 cm, and the mean annual a real rainfall was 122 cm. On the average, drainage exceeded supplemental water use in the months of June to September while from December to March drainage and supplemental water use were equivalent. Supplemental water use exceeded drainage in the months of October, November, April, and May. High drainage occurred in June and September; smallest drainage was in February. On the average, the highest supplemental water use occurred in May and November. The 10-year return period of annual drainage during wet and dry cycles were 60 cm and 38 cm per year, respectively. The semi-monthly drainage coefficient of variation (cv) is above 100 percent for the period from the second half of October to end of April. The cv is lower than 100 percent for the remaining season (wet season). The purpose of this paper is to present the magnitude, temporal, and frequency distribution of drainage runoff generation and supplemental water use in the EAA basin. Information on statistics of drainage will contribute to the optimization of the design and operation of drainage water treatment systems.     

STATISTICAL LOW FLOW ESTIMATION USING GIS ANALYSIS IN HUMID MONTANE REGIONS IN PUERTO RICO1

ABSTRACT: Statistical analysis of watershed parameters derived using a Geographical Information system (GIS) was done to develop equations for estimating the 7d–10yr, 30d–10yr, and 7d–2yr low flow for watersheds in humid montane regions of Puerto Rico. Digital elevation models and land use, geology, soils, and stream network coverages were used to evaluate 21 geomorphic, 10 stream channel, 9 relief, 7 geology, 4 climate, and 2 soil parameters for each watershed. To assess which parameters should be used for further investigation, a correlation analysis was used to determine the independence and collinearity among these parameters and their relationship with low flows. Multiple regression analyses using the selected parameters were then performed to develop the statistical models of low flows. The final models were selected in the basis of the Mallow Cp statistic, the adjusted R², the Press statistic, the degree of collinearity, and an analysis of the residuals. In the final models, drainage density, the ratio of length of tributaries to the length of the main channel, the percent of drainage area with northeast aspect, and the average weighted slope of the drainage were the most significant parameters. The final models had adjusted standard errors of 58.7 percent, 59.2 percent, and 48.6 percent for the 7d–10yr, 30d–10yr, and 7d–2yr low flows respectively. For comparison, the best model based on watershed parameters that can be easily measured without a GIS had an adjusted standard error of 82.8 percent.     

SNOWPACK CHANGES RESULTING FROM TIMBER HARVEST INTERCEPTION,REDISTRIBUTION, AND EVAPORATION1

ABSTRACT: Three processes were examined as causing snowpack changes in forest clearings. Two of the three contribute to increases and one counteracts by reducing snowpack. The two that increase snowpack are redistribution and decreased loss to interception. Snow evaporation from a clearing counteracts snowpack increases. Research has indicated that as vegetation density increases, so too does the loss to interception. As snow in the canopy reaches the limit that the canopy can hold (the threshold amount) evaporation increases. Aerodynamics of the forest canopy were studied as well. As timber is cut, wind patterns are disturbed, creating disruptions in the wind velocity gradient depositing snow in openings. This redistribution leads to an increased snow water equivalent and augments runoff. Snow evaporation was shown to increase proportionally with opening size. Evaporation offsets the water yield gains derived from forest cut. It was found that this offset is inclusive to the measurements of water yield changes in experimental forests. An optimal size of harvest block may be five tree heights in width as suggested by numerous studies.     

AN OBJECTWE TEST FOR HYDROLOGIC SCALE1

ABSTRACT: Improving the reliability of parametric hydrologic models (sometimes called cenceptual rainfall-runoff models) in the continuous simulation of runoff from ungaged catchments has been frustrated by difficulties in estimating model parameters from catchment characteristics. An underlying problem is that these models use parameters to represent catchments as a whole, whereas data on catchment characteristics are collected at multiple field locations and are difficult to transform into one measure of collective impact. Subdividing the catchment and calibrating a stochastic parametric model to estimate distributions for the parameters that covered the range of observed streamflow values was found to improve the simulations. This paper presents an optimization of the amount of subdivision to use in simulation with a version of the Stanford Watershed Model using available climatological data. The calibration process assumes that catchment heterogeneity introduces errors that can be reduced by calibrating parameters as spatial distributions rather than single values. Calibrations for three diverse small gaged catchments located in California and in Virginia found the optimal number of subdivisions to range from 4 to 25 and the optimal scale to range from 0.3 to 2.1 mi².     

SIMULATION OF SEDIMENT AND PLANT NUTRIENT LOSSES BY THE CREAMS WATER QUALITY MODEL1

ABSTRACT: CREAMS was applied to a field-sized watershed planted to cotton in the Limestone Valley region of northern Alabama. The field was cultivated for three years with conventional tillage (CvT) followed by three years of conservation tillage (CsT). CREAMS is composed of three components: hydrology, erosion, and chemistry. Surface runoff and losses of sediment, N and P were simulated and results were compared with the observed data from the watershed. Curve numbers recommended in the CREAMS user's guide were not adequate for the watershed conditions. The hydrology submodel improved runoff simulation from CvT and CsT when field-data based curve numbers were used. The erosion submodel demonstrated that CsT reduced sediment loss more than CvT, even though CsT had higher runoff than CvT. The nutrient submodel based on the simulated runoff and sediment underpredicted N loss for both CvT and CsT. This submodel, however, accurately predicted P loss for CvT, but underpredicted for CsT (50 percent lower than the observed). The results of CREAMS simulation generally matched the observed order of magnitude for higher runoff, lower sediment, and higher N and P losses from CsT than from CvT.