Seasonal water balance of an Ozark hillslope

Abstract Analysis of field water balance components provides information necessary to minimize the risk of offsite movement of contaminants from crop production practices or animal manure applications. The objective of this study was to determine the timing and amount of surface runoff and drainage from the root zone for a hillslope in the Ozark Highlands of US. A 0.4 ha watershed with slopes of 8–20% having tall fescue ( Festuca arundinacea Schreb.) cover was established in northwestern Arkansas (35°56′W, 93°51′N). Continuous measurements of water balance parameters were made from June 1997 to August 1998. Soil water drainage was estimated as the residual of weekly water balance calculations. Runoff occurred in response to three precipitation events in the winter of 1998 and totaled 30.6 mm of water or 2.6% of the 1185 mm of precipitation that fell at the site during the study period. Storms of comparable or greater intensity during other seasons failed to produce runoff, a result that was likely due to dry soil conditions and taller grass canopy. Drainage through the root zone totaled 117 mm and occurred primarily during an 83-day interval in the winter of 1998. The water balance was dominated by evaporation, which accounted for 91% (1080 mm) of the precipitation. Tall fescue was capable of sustaining relatively high evaporation rates between infrequent summer rains thereby dewatering the soil profile, which was not replenished until winter. Delaying spring animal manure applications in the Ozarks until evaporation has increased and the soil profile has begun to dry would decrease the risk of offsite transport of potential contaminants contained in the manure.