1a. Objectives (from AD-416)
This is to provide partial funding to support the effort of modifying and evaluating the soil erosion components of the SWAT, and collecting and dating reservoir sediment using nuclear isotopes for model evaluation.
1b. Approach (from AD-416)
In SWAT, a basin is divided into sub-basins, and sub-basins are further divided into Hydrological Response Units (HRU). The soil erosion is calculated using MUSLE separately and independently for each HRU without linkage between HRUs, which is oversimplified and sometimes unrealistic in terms of water and sediment routing. The hydrological and soil erosion components will be restructured to allow linkage between HRUs (i.e., allow water and sediment to discharge to another HRU). The modified model will be evaluated using reservoir sediment data, along with historical land use and climate of selected watersheds. Multiple sediment cores will be taken from selected reservoirs, and basic physical (e.g., particle size distribution) and chemical properties (e.g., mineral oxides) of sediment will be measured by depth. Nuclear isotopes including 137Cs, 210Pb, 226Ra, and 232Th will be measured using a gamma spectrometer. The measured isotope ratios will be used to estimate how much soil loss is from overlands and how much is from gullies. These sediment source data, rarely measured in field, will be used to evaluate the modified SWAT. The validated model will be used to evaluate the effects of land use changes and climate variability on water resource and soil erosion in the selected watersheds.
3. Progress Report
The Bull Creek watershed in the upper reaches of the Fort Cobb watershed was selected for detailed study of this project. The SWAT input data of the Bull Creek watershed were collected and compiled. The soil survey spatial and tabular data (SSURGO 2.2), 2010 National Ag. Imagery Program Mosaic, 10-m Digital Elevation Model (DEM) from National Elevation Dataset, and 2001 National Land Cover Data were downloaded from the USDA web site. The DEM were used to delineate the Landform units such as summit, shoulder, back slope, foot slope, and channel and to calculate Topographic Wetness Index for use in designing sampling scheme along hillslopes and across the watershed using computer programs that were downloaded from the ESRI web site. To assist designing an efficient sampling scheme, soil loss in the watershed was estimated using the USLE (Universal Soil Loss Equation) using the downloaded data. A preliminary sampling scheme as a GIS layer was developed based primarily on topographic Wetness Index and further stratified by land use and soil type. Soil samples will be taken in July and August of 2011 using the sampling scheme. The ADODR monitored progress of the project through meetings, e-mails, and telephone conversations.