Extracting Pasture Evapotranspiration Parameters from Proximal Sensing and Mathematical Modelling

Abstract

<p>Knowledge of crop evapotranspiration is crucial for irrigation decision making. An appropriate, user-friendly and time-efficient means of inferring such information is therefore essential. In this study, a closed hemispherical chamber was instrumented, calibrated and deployed in the field for measuring actual evapotranspiration of a vital pasture species, Tall Fescue (<i>Festuca arundinacea</i>). The pasture crop coefficient (K_c) was calculated from the measured instantaneous evapotranspiration and reference crop evapotranspiration (ET_o) for a range of growth stages. Also the relationship between K_c and Normalized Difference Vegetation Index (NDVI) as measured using an active optical sensor was established. Using the FAO dual crop coefficient approach and the hemispherical chamber, a technique for partitioning evapotranspiration components was developed. The components of evapotranspiration in terms of basal crop coefficient (K_cb) and soil evaporation coefficient (K_e) were expressed relative to canopy NDVI and Leaf Area Index (LAI). A theoretical model for estimating transpiration was also developed by scaling up stomatal conductance to canopy level in a controlled glasshouse environment. The model was validated against the measured transpiration.</p>