Measuring evapotranspiration in urban irrigated lawns

Conservation of water is becoming increasingly critical in many metropolitan areas. The use of automated irrigation systems for the maintenance of lawns and landscapes is rising and these systems are typically maladjusted to apply more water than necessary, resulting in water wastage. Provision of accurate estimates of actual lawn water use may assist urbanites in conserving water through better adjustment of automatic irrigation systems. Micrometeorological methods may help determine actual lawn-water use by measuring evapotranspiration (ET) from urban lawns. In October 2009, four small tripod-mounted weather stations (tripods, five total) were deployed in eight residential yards in the northwestern portion of the city of Manhattan, KS, USA. Each tripod was instrumented to estimate reference crop evapotranspiration (ET_o) via the FAO-56 method. During tripod deployment in residential lawns, actual evapotranspiration (ET_actual) was measured within five km from well-watered turfgrass using a stationary, trailer-mounted eddy covariance (EC) station. The fifth tripod was deployed in the source area of the EC station to estimate ET_o in conjunction with tripods in the lawns (i.e., to serve as a reference). Data from EC allowed for computation of a so-called lawn coefficient (K_c) by determining the ratio of ET_o from the tripods in residential lawns to ET_o from the EC station (ET_o,EC); hence, K_c = ET_o,tripod / ET_o,EC. Using this method, ET_actual can be estimated for individual lawns. By averaging all of the lawn-specific coefficients (K_c,i), a much larger area (in this case NW Manhattan) could be represented by one overall lawn coefficient (K_c). Using data corresponding to days where optimal conditions portended more accurate ET_o estimates by the tripods (i.e., southerly winds, temperatures well above freezing, and sunny), coefficients specific to individual lawns ranged from K_c,i = 0.49 – 0.79 with the exception of one outlier (0.21). The overall average K_c was 0.639 for optimal conditions. Estimates of ET_actual for the entire NW portion of Manhattan are then found by adapting the overall lawn coefficient K_c = 0.639 to the actual ET measured by the EC station (ET_actual,EC). Similarly, multiplying ET_actual,EC by individual K_c,i will also yield ET_actual, however this method is only applicable on a yard-to-yard basis. Determining ET_actual for the lawns in our study by using K_c was to within ±10.9% of ET_actual calculated using the lawn-specific coefficients (K_c,i). Cumulative ET_actual,EC for the duration of each lawn deployment was on average 10.9 ± 1.9 mm, while cumulative ET_actual in residential lawns using K_c = 0.639 averaged 6.9 ± 0.8 mm; lower windspeeds and shaded areas in home lawns contributed to lower ET in town compared with the EC site. When calculating average cumulative ET_actual with K_c,i from individual lawns, ET_actual was 6.2 ± 1.8 mm. When considering differences between the EC site and individual lawns, which included variations in lawn maintenance and microclimates, estimates of in-town ET found by correcting EC data using an overall lawn coefficient were good. Moving the EC station closer within the boundaries of the city would likely improve the lawn coefficient to be more representative of individual tripods/yards.