Hydrologic and water quality models are very sensitive to input parameter values, especially precipitation input data. With several different sources of precipitation data now available, it is quite difficult to determine which source is most appropriate under various circumstances. We used several sources of rainfall data in this study including single gauge rainfall data located outside the watershed boundary, and next generation radar (NEXRAD) rainfall data with different corrections, to examine the impact of such sources on Soil and Water Assessment Tool (SWAT) model streamflow predictions tor a 50 km 2 watershed located in the coastal plain of Maryland. For a watershed of that size with annual average precipitation of 43 inches, at least 3 rain gauges within the watershed would reduce the percentage error in measured average watershed rainfall amounts to less than 23% (for 0.5 inch storm events). The larger the amount of storm rainfall the less error was associated with its measurement. Model simulation results indicated that distance and location of the single rain gauge located outside the watershed boundary has a significant impact in simulating hydrologic and water quality response of the watershed in the temperate region of Maryland. In the absence of a spatially representative network of rain gauges within the watershed, NEXRAD data produced more accurate estimates of streamflow than using single gage data. This study concludes that one has to be mindful of the source and methods of interpolation of rainfall data for input into hydrologic and water quality models if simulation accuracies are desired.
Wetlands are an integral part of many agricultural watersheds. They provide multiple ecosystem functions, such as improving water quality, mitigating flooding, and serving as natural habitats. Those functions are highly depended on wetland hydrological characteristics and their connectivity to the downstream waters. However, wetland hydrology has been poorly understood at the watershed scale. In this study, we simulated the Soil and Water Assessment Tool (or SWAT) model along with the inclusion of a Riparian Wetland Module (RWM) at Tuckahoe Creek, a sub-watershed within the Choptank River watershed on the Coastal Plain of the Chesapeake Bay watershed. The RWM, a SWAT extension module, was adopted to better simulate interactions between riparian wetlands and nearby streams. The SWAT-RWM was calibrated and validated against observed streamflow collected at the outlet of the watershed and then applied for assessing wetland hydrological benefits. The poster will demonstrate the hydrological benefits of wetlands to stabilize overall flow pattern and reduce peak flow at the storm event. The outcomes of this study will contribute to the enhanced understanding of the hydrological role of wetlands in the watershed. © 2016 American Society of Agricultural and Biological Engineers. All rights reserved.
Elevated C02 concentration, temperature, and change in precipitation patterns driven by climate change are expected to cause significant environmental effects in the Chesapeake Bay Watershed (CBW). Although the potential effects of climate change are widely reported, few studies have been conducted to understand implications for water quality and the response of agricultural watersheds to climate change. The objective of this study is to quantify changes in hydrological processes and nitrate cycling, as a result of climate variability, using the Soil and Water Assessment Tool (SWAT) model. Specifically we assessed the performance of winter cover crops (WCC) as a means of reducing nutrient loss in the realm of climate change and evaluate its impacts on water quality at the watershed scale. WCC planting has been emphasized as the most cost-effective means for water quality protection and widely adopted via federal and state cost-share programs. Climate change data were prepared by modifying current climate data using predicted mean temperature and precipitation change for the future periods (2070-2099) predicted by four global climate models. Current CO2 concentration, temperature, and precipitation increased by 850 ppm, 4.5 °C, and 23%, respectively. Although temperature increase reduced the water and nitrate loads, nitrate loads were found to increase by 40% under baseline land management and WCC were found to be less effective at reducing nitrate (nitrate increased by 4.6 kg/ha). Therefore agricultural conservation practices are likely to be even more important in the future, but acreage goals may need to be adjusted to maintain baseline effects.
Concern over chemical loadings to unconfined aquifers and into the surface water sources through drain tiles and subsurface groundwater flow has directed researchers to focus on the pathways that speed up the pollutant arrival to such sources. Preferential flow of water and chemical transport though porous media has attracted the attention of scientists and engineers working in the environmental field. Research has indicated that structured soils promote bypass flow (a form of preferential flow induced by macropores formed due to shrinking and swelling of soils) that results in fast movement of solutes, whereas piston flow is mostly responsible for the flow of water and solutes in nonstructured homogeneous (e.g., homogeneous sand-textured) soils (Skopp, 1981; Schumacher, 1864; Bergstrom and Shirmohammadi, 1999). Nieber (2001) stated that preferential flow includes macropore flow, gravity-driven unstable flow, heterogeneity-driven flow, oscillatory flow, and depression-focused recharge. Flow systems such as fingering caused by a sequence of texturally different layers (Hill and Parlange, 1972), hydrophobicity (Ritsema et al., 1983), and funnel flow due to texturally different lenses (Kung, 1990) are examples of preferential flow mechanisms. Such multiple transport behavior has created a multitude of difficulties in modeling solute leaching in vadose zone. © 2005 by CRC Press.
Agricultural and Forest Meteorology (01681923)33(2-3)pp. 225-238
A field evaporation-drainage study was conducted to compare three methods of predicting evaporative losses from a bare soil. Two of the methods (modified Penman combination and Idso-Jackson) are dependent only on measurements of atmospheric parameters whereas the third method (plane of zero flux) is dependent only on measurements of soil parameters. A Captina soil profile was wet up and allowed to dry by evaporation and drainage. For the initial two days after infiltration ceased all three methods predicted similar evaporative losses. Differences between the three methods occurred when the soil moisture content at the soil surface controlled the evaporation rates. Under the three drying conditions the three methods behaved somewhat differently in the prediction of the amounts of water evaporated from the soil surface. Lower losses by evaporation were predicted by the Idso-Jackson and zero-flux methods. In the case of the Idso-Jackson method this result was attributed to the influence of clouds on albedo, the impact of wind and the importance of albedo in the predictive equation. For the zero-flux method the decrease in evaporation was due to lower soil water contents and matrix potentials near the surface which resulted in lower transport rates of water to the surface. © 1984.
The use of tile drains for alleviating soluble salt accumulation on silt loam soil was investigated during 1984. Although the chemical analyses of the floodwater and tile drainage water were very similar suggeting that the floodwater was moving to the tile drain, the overall results so far indicate that this is not a feasible solution owing to lack of significant drainage. Application of DRAINMOD utilizing soil and weather data from Arkansas showed no significant effluent from the tile drains for our experimental site during rice production. This was attributed to the extremely slow saturated hydraulic conductivity values for this particular soil. However, more observations (concerning the operation of the tile field) are needed before it can be concluded that tile drain fields are not a viable solution to the problem.
IEEE Transactions on Geoscience and Remote Sensing (01962892)22pp. 394-405
Results are presented of an experimental program to determine the functional dependence of the microwave reflectivity of nonvegetated soil surfaces upon volumetric soil moisture and matric potential. A combination evaporation-drainage field experiment was conducted on a bare Captina silt loam with reflectivity, soil moisture content, and matric potential monitored for extended time periods. Results show that for a restricted pressure range (approximately -0.05 to -0.75 bar) there is excellent linear correlation between the log of bistatic reflectivity and both volumetric moisture content and matric potential. Layering effects due to steep moisture content (and matric potential) gradients in the profile are demonstrated to have two distinct and significant effects on the reflectivity response. At near saturation of rough surfaces a very thin dry surface layer appears to modify the effective roughness. This leads to a saturation of reflectivity at high moisture contents. As the surface proceeds to dry further, deeper layers produce coherent interference patterns in the reflectivity response, particularly at the higher frequencies. © 1984 IEEE
Sadeghi, A.,
Hancock g.d., ,
Waite w.p., W.P.,
Scott h.d., ,
Rand j.a., Water Resources Research (00431397)20(7)pp. 927-934
Laboratory and field experiments were conducted to investigate the ability of microwave remote sensing systems to detect the moisture status of a silt loam soil exhibiting abrupt changes in moisture content near the surface. Laboratory soil profiles were prepared with a discontinuous moisture boundary in the subsurface. Reflectivity measurements of these profiles were made with a bistatic reflectometer operating over the frequency ranges of 1–2 and 4–8 GHz (wavelength ranges of 30–15 and 7.5–3.75 cm, respectively). These measurements exhibited a well‐developed coherent interference pattern in good agreement with a simple two‐layer reflectivity model. Field measurements of bare soil surfaces were conducted for initially saturated profiles and continued for extended periods of drying. During drying, coherent interference patterns similar to those observed in the laboratory were detected. These appear to be due to steep moisture gradients occurring between drying layers near the surface. The field results were modeled by a five‐segment linear moisture profile with one or two steep segments and a multilayer reflectivity program. Agreement between model and field response over the frequency range was used to estimate the depth of drying layers within the soil. These depths were monitored over the second and third drying cycles. Formation of the drying layers under field conditions appears to be influenced by drying time, tillage, and evaporative demand. In any case, it appears that the coherent effects caused by nonuniform moisture profiles may substantially affect the reflectivity of even rough soil surfaces. Copyright 1984 by the American Geophysical Union.
