Estimating preseason irrigation losses by characterizing evaporation of effective precipitation under bare soil conditions using large weighing lysimeters
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2016. Irrigation from the Ogallala aquifer is used to supplement insufficient precipitation for agricultural crop production in the semi-arid Texas High Plains. Decreased pumping capacity has compelled many producers to "pre-water" fields to field capacity prior to planting to hedge against pumping limitations later in the season. However, the direct measurement of evaporative losses from preseason irrigation of bare soil is not commonly studied. The quantification of evaporative losses from effective precipitation, or the net amount of water that infiltrates into the soil following a precipitation event, can be used as a surrogate for estimating losses from preseason irrigation. We identified 35 precipitation events that occurred over lysimeter fields under fallow conditions in 2002, 2005, and 2009. Events were categorized into four bins of precipitation magnitude ranging from 3 mm to 35 mm. Subsequent evaporation was measured for a period of up to seven days following rainfall events using large weighing lysimeters at the USDA-ARS Conservation and Production Research Laboratory in Bushland, TX. An exponential decay function was used to characterize bare soil evaporation using maximum cumulative measured evaporation (ECmax), soil water transfer constant (k), and cumulative grass reference evapotranspiration (ETCos). The wide range of ECmax values and k values demonstrated the sensitivity of evaporative losses to both antecedent soil water content and evaporative demand. We also present measured average daily evaporation values for a range of evaporative demand regimes for each precipitation bin. From data analyzed in this study, nearly all of the water from precipitation events of 10 mm and less were lost to evaporation within the following day under moderate to high grass reference evapotranspiration (ETos) conditions. Nearly all water from precipitation events between 20 and 30 mm was lost to evaporation between three to four days following the event under similar evaporative demand. The considerable potential evaporative losses from preseason irrigation call into the question the prudence of the preseason irrigation, particularly for regions with limited groundwater resources.
