Estimating urban landscape evapotranspiration

There have been attempts to empirically estimate evapotranspiration (ETl) from landscape vegetation to develop irrigation schedules, but the methods are often based on guessing rather than science. Therefore, an Excel application program (LIMP) was recently written to help landscape professionals develop a more scientifically based model. The model uses monthly means of daily data and the Penman-Monteith (PM) equation to provide a regional reference evapotranspiration (ETo) estimate. Data from the landscape site are also used with the PM equation to estimate the local microclimate ETo (ETm). Then a microclimate coefficient (Km) equals the ratio of ETm over ETo. A vegetation coefficient (Kv) is calculated as ET from well-watered vegetation (ETv) divided by ETm. A plant density coefficient (Kd) is estimated from an algorithm for deciduous trees using the percentage ground cover to account for sparse canopies. Then a stress coefficient (Ks) is used to adjust for water stress effects on ET. ETo rates and the number of rainy days per month are used to estimate the baseline bare-soil evaporation rate (Es) and an evaporation coefficient (Ke), which equals Es divided by ETo. A smooth curve fitting technique is used to estimate daily coefficients from the monthly values and the daily landscape coefficient (Kl) is calculated as the product of Km, Kv, Kd and Ks but not less than Ke. Finally, ETl is calculated as the product of ETo and Kl. This paper will present the LIMP model and its possible applications.