Impact of Weather Station Fetch Distance on Reference Evapotranspiration Calculation

Reference evapotranspiration (ET_o) has been used for irrigation scheduling and water management of urban landscapes from the publicly accessible weather networks by water governing districts and homeowners alike. However, most of these weather stations are located on small grass surfaces and does not meet the minimum fetch distance as required by the standard. The four parameters used in the ET_o calculations are temperature (T), relative humidity (RH), solar radiation (R_s) and wind speed (U). The solar radiation is the least influenced parameter by the local surface condition, while the wind speed is the most affected factor. Allen proposed that when the wind speed is higher than 2 m/s, the temperature and relative humidity measurements are not conditioned by the grass surface surrounding the station with adequate fetch as required by the standard, but by the fetch distance upwind of the weather station. When the wind speed is less than 2 m/s, mathematical equations have been developed to evaluate the influence of the upwind fetch distance on T and RH measurements. These equations show that the impact factor, fraction of vapor and sensible heat flux densities at height z contributed by fetch over upwind distance, varies with measurement height and fetch distance. The objectives of this study were to estimate the appropriate RH/T sensor height for various fetch distances using Gash's footprint model in urban areas. The impact of sensor height and fetch distance on reference ET calculations using the ASCE-EWRI Penman-Monteith equation with a grass reference were examined. In this experiment, five weather stations, one located on an ideal large grass reference surface in an agricultural region and four stations on non-ideal small grass surface in urban areas, were used to examine the impact of fetch. For each station, there are three RH/T sensors, located at approximately 0.5, 1.0 and 1.5 m. Solar radiation and wind speed are also measured at each station. The daily ET_o showed no difference at the three RH/T sensor heights, indicating that the ambient environment had little influence on the RH/T measurement in humid climate, even under non-ideal reference condition. However, more data need to be collected to assess the effect of fetch over an annual cycle.