1a. Objectives (from AD-416):
This is to provide partial funding to support the effort of evaluating the soil erosion components of the SWAT, finger-printing sediment sources and spatial distributions, and collecting and dating reservoir sediment using nuclear isotopes.
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). Radionuclide 137Cs and other geochemical tracers will be used in selected watersheds for model evaluation. The modified model will also 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 main effort of the current year is to complete field sampling in the selected Bull Creek watershed and the subsequent laboratory processing of the field samples. About 70 surface soil samples and channel bed materials were sampled in the Bull Creek watershed. The surface soil samples were classified according to soil type and land uses, and the channel bed sediment samples were representative for all the stream orders. Gully and stream bank samples were also taken at locations where erosion is currently active to represent subsoil sediment sources. Apart from soil samples, runoff sediment samples were collected at about 10 sites across the watershed. In addition, sediment samples from several ponds in the watershed were taken. All soil and sediment samples were preprocessed in the lab (ground, dried, and sieved through a 2-mm sieve). The 72 surface samples and 42 runoff sediment samples were extracted with nitric acid using the EPA 3050 method for ICP-MS chemical element analysis for potential identification of sediment source tracers or fingerprints. In addition, about 50 soil samples from 9 transects in a rangeland sub-watershed were collected for 137Cs analysis to characterize the soil redistribution in the area. The samples were shipped to Beijing Normal University for gamma spectrometer analysis. Mass balance models and Mixing models will be used to calculate the soil redistribution and the sediment contributions, respectively.