Title: AnnAGNPS: Baseflow Feature Authors
|Yuan, Yongping - UNIVERSITY OF MISSISSIPPI|
|Theurer, Fred - USDA-NRCS|
Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: May 1, 2008
Publication Date: June 30, 2008
Citation: Yuan, Y., Bingner, R.L., Theurer, F.D. 2008. AnnAGNPS: Baseflow Feature. ASABE Annual International Meeting. St. Joseph, Michigan. CD-ROM. Paper No. 084243. Interpretive Summary: The USDA Annualized Agricultural Non-Point Source (AnnAGNPS) watershed conservation planning model has proven to be an effective tool for evaluating watershed management efforts. Enhancements needed to assess baseflow contribution to stream reaches within watershed models are described in this study. Baseflow occurs when groundwater appears as part of stream flow and can be a significant contributing part of the entire basin’s water resources. When watershed models are used for long-term water resource analysis and planning, contribution of baseflow should not be ignored. Groundwater flow processes and governing equations were reviewed and introduced in the paper. Subsequently, groundwater flow processes were incorporated into AnnAGNPS, by developing an approach to improve the watershed system’s water balance with an unconfined aquifer, including groundwater recharge, discharge (baseflow) and pumping components. Evapotranspiration and deep percolation from the aquifer were neglected. Percolation below the soil profile that was previously assumed to be lost from the system was considered as resources for groundwater recharge. As a result of these enhancements, chemicals transported by baseflow processes can be evaluated for their contribution to watershed water quality for use by state and federal action agencies.
Technical Abstract: The Annualized Agricultural Non-Point Source Pollutant Loading model is a watershed scale, continuous simulation, daily time step model that is currently utilized in many locations of the U.S. by the Environmental Protection Agency, Natural Resources Conservation Service, and others to estimate the impact of best management practices on non-point source pollution. The model has many unique and powerful capabilities, but prior to AnnAGNPS version 4.0, it did not simulate baseflow. Percolation below the soil profile was simply treated as a loss to the system and ignored. Flow interactions between the unconfined aquifer and stream channel systems were not considered. Subsequently, groundwater flow processes was incorporated into AnnAGNPS and is described in this paper. Groundwater recharge is determined using an exponential decay weighting function proposed by Venetis and used by Sangrey et al. in a precipitation/groundwater response model. This model is utilized to account for the time delay through the vadose zone and into the aquifer once the water exits the soil profile. Baseflow is determined using an adaptation of Hooghoudt’s equation. Baseflow in streams occurs only when the water table is higher than the water level in the stream channel. Sensitivity analysis showed that the sensitivity of groundwater recharge to the delay time is limited to a few days. The sensitivity of baseflow to the thickness of the aquifer is also limited. After certain depth of the aquifer, the baseflow does not change with the increase of the thickness of an aquifer. Results of “proof of concept” simulations showed that the accumulated baseflow followed the accumulated recharge and/or deep percolation after certain time, and the length of time depends on aquifer properties. Model evaluation was performed by comparing AnnAGNPS simulated total runoff with field observed total runoff at the outlet of the Goodwin Creek watershed. Comparisons between the simulated and observed monthly runoff at the outlet of the watershed produced a Nash-Sutcliffe coefficient of efficiency of 0.72, the relative error of 0.17 and the Willmott index of agreement of 0.92.