Toward setting realistic expectations for agricultural management practices based on water flow paths and lag times.

Abstract

Agriculture makes up over 40% of all the land area in the United States and influences watersheds both large and small. As water moves through a watershed of any size it moves over the land as runoff into surface waters, infiltrates into the soil to groundwater, and eventually discharges to surface water. In some systems such as forests lateral subsurface flow is dominate and bypasses the groundwater component. Whatever the system the movement of water through the watershed drives the movement of agricultural chemicals. Best Management Practices (BMPs) are used to partially control the movement of water. The goal of this work is to help set realistic expectations of BMPs based on a holistic approach to watershed modeling. To do this, three process-based models were used (GeoWEPP, LEACHM, and MODFLOW) to simulate the spatiotemporal movement of water through three compartments (surface water, vadose zone, and groundwater) of the watershed. Spatial characteristics of a particular field such as its topographical location, distance from stream, and its distance to the groundwater are all factors into the lag time, quantity, and quality of water reaching the stream. Morgan Creek in Maryland was used as an example watershed to illustrate how spatial and temporal differences affect management decisions and expectations. GeoWEPP and LEACHM were used to investigate runoff and infiltration while MODFLOW was used for ground water lag times. Runoff and infiltration values were investigated with slopes ranging from 0-100%, varying management practices, and varying soil types. Runoff and sediment yield values resulting from eight different management practices (Corn – fall mulch, Continuous Corn – no till, Corn/soybeans/wheat/alfalfa – no till, Corn/soybeans/wheat/alfalfa – conventional till, fallow – tilled, alfalfa with cutting, 20 year old forest, and five year old forest) were compared to each other, as were the 3 dominant soil types (Mattapex, Sassafras, and Butlertown). These differences were looked at both spatially and temporally. Subsurface (vadose zone plus ground water) lag times within a single field reveal that within that single field subsurface lag time range from 9 years up to 30 years. A holistic understanding the as to water’s movement through all compartments the within the watershed will improve the decisions being made and the expectations placed on those decisions.