Seasonal variation in the energy and water balances of an Ozark grassland

Movement of water, either as surface runoff or groundwater recharge, is the primary mechanism of transport for agrochemicals in the terrestrial environment. The objective of this study was to monitor seasonal changes in surface energy and water balance components at an Ozark Highland site with endophyte-infected tall fescue (Festuca arundinacea Schreb.) cover. Two 0.40-ha watersheds were established in the Boston Mountains of northwestern Arkansas (35° 56' W, 93° 51' N, 460 m above MSL) on Enders-Leesburg stony loam soil with 8 to 20% slopes. Precipitation and surface runoff were measured beginning in 1994. Bowen ratio instrumentation and neutron moisture meter access tubes were added in 1997 to determine energy balance components and solve the surface water balance. Continuous energy and water balance measurements were made from June of 1997 through July 1998. The surface water balance was dominated by evaporation as, of the 1185 mm of precipitation measured over this interval, 91% (1080 mm) was lost to evaporation. Surface runoff and groundwater recharge occurred only during the winter months and accounted for 31 and 117 mm, respectively, of the precipitation. Evaporation occurred at near potential rates, as predicted by the Priestley-Taylor equation, from September to April and between 60 and 80% of potential during the summer (May-August). Analysis of the summer evaporation data indicated that the tall fescue exhibited drought tolerance characteristics, possibly due to a high degree of endophyte infection, and was capable of extracting soil moisture from deep in the root zone. The timing of surface runoff and groundwater recharge were the result of low evaporation rates that allowed the soil to saturate during the winter rather than from intense storm events during the summer.