Effect of Lawn Microclimates On Estimates of Evapotranspiration From Atmometers.

An atmometer is an inexpensive tool used to measure evapotranspiration (ET) in situ. The effects of microclimates in urban lawns on the performance of atmometers are not well documented. The objective of this study was to compare ET estimates from atmometers placed in different microclimates in residential lawns and in an open sward to the FAO-56 Penman-Monteith (FAO56) empirical ET model calculated from weather data obtained in the same location. The study was conducted in six home lawns in 2010 and one in 2011 in Manhattan, KS, and four home lawns in Wichita, KS in 2011. A weather station and atmometer were positioned in an open sward of turfgrass near each city during each measurement period in Manhattan and Wichita, KS. Bellani plate atmometers were placed next to portable weather stations that were placed at two different microclimates in each lawn. Weather stations recorded temperature, net radiation, relative humidity, and wind speed data used to calculate daily FAO56 ET. Mean ET was 4.26 mm d^-1 for the atmometers, which was 7% greater than FAO56 ET (3.97 mm d^-1). Atmometer ET (4.92 mm d^-1) from the open sward weather station only was 13% less than FAO56 ET (5.68 mm d^-1). However, within home lawn microclimates, atmometer ET (3.94 mm d^-1) was 25% greater than FAO56 ET (3.16 mm d^-1). Regression analysis of FAO56 ET versus atmometer ET showed that ET from the open sward weather station (r²=0.57) and home lawn microclimates (r²=0.82) had slopes of 1.04, and 0.88 respectively. This indicates that within home lawn microclimates the difference between atmometer ET and FAO56 ET will become greater as ET increases, while open sward ET measurements will be nearly equal. Based on these findings, greater differences between FAO56 and atmometer may be expected within home lawn microclimates.