Measurement of Unirrigated Turfgrass Evapotranspiration Rate in the Red River Valley

Abstract

Core Ideas Soil moisture is needed in defining well‐watered turfgrass condition in evapotranspiration measurement. Deep soil moisture supports turfgrass growth when surface soil moisture is not sufficient. The ETa can be estimated from 0.96 ETo using the ASCE‐EWRI standardized method for well‐watered turfgrass. The ASCE‐EWRI standardized reference ET equation can be used for water management of turfgrass in northern cool climates and during drought periods. Turfgrass actual evapotranspiration (ETa) measurements are critical for water management and irrigation scheduling. With no historical ETa measurements in eastern North Dakota, turfgrass ETa rates were measured with the residual method using eddy covariance instrumentation and two arrays of soil moisture sensors on unirrigated turfgrass under natural conditions in the 2011, 2012, and 2013 growing seasons. An on‐site weather station provided weather data to calculate the standardized grass‐based reference evapotranspiration (ETo) (Allen et al., 2005). The daily ETa/ETo ratios were screened using the criteria of soil moisture ≥50% of available water for the top 30 cm of the root zone, rain amounts ≤10 mm, and a recovering period after drought. The screened monthly average ETa/ETo ratios for the unirrigated turfgrass were 1.03, 0.98, 0.94, 0.90, 0.82, and 1.18 from May to October. The mean ETa/ETo ratio for the entire growing seasons was 0.96, implying that the American Society of Civil Engineering–Environmental and Water Resource Institute ETo method was valid for guiding the turfgrass ETa calculation even in unirrigated and cold climate conditions. Because this is the first reported study on ETa measurement of a turfgrass site, the limited data can provide a baseline on water management for turfgrass under various weather conditions in this region. The results indicated that a monthly refinement of ETa/ETo values might be required to maintain the landscape turfgrass quality more precisely in terms of water management.