Rapid measurement of pasture evapotranspiration components using proximal sensors

Background: Evapotranspiration (ET) is the total amount of water released by a crop or pasture canopy in the form of Transpiration (T) and Evaporation (E). ET accounts for up to 96% of the water loss depending on the types of vegetation cover and climatic conditions (Wilcox et al., 2003). Transpiration is related to the productivity of crops and pasture; whereas, evaporation is the loss of water directly from soil surface. Estimating separately the components of ET in the field is challenging yet it is highly significant in terms of improving crop water use and irrigation efficiency, since a anywhere between 30 and 80% of the water flux can be associated with the all important evaporation component (Wilcox et al., 2003). Sap flow monitoring, using micro-lysimeters or isotopic analysis are the usual options for separately measuring transpiration and evaporation in plants but these are incompatible with in-situ field deployment. In this study a portable and convenient method for separately determining the evapotranspiration components has been developed. When coupled with widely-used active optical sensors, the device can be used to develop, in-situ, relationships between spectro-optical indices such as NDVI and evapotranspiration coefficients (Kc, Kcb and Ke) that characterize actual ET water loss relative to evaporative demand.

Methods: A 0.5 m diameter, portable, perspex dome was equipped with temperature and humidity sensors and a circulating fan to calculate the rate of vapour accumulation inside the chamber when placed over a target pasture canopy (Festuca arundinacea var. Dovey). When deployed it takes less than a minute to measure the actual evapotranspiration rate (Alam et al., 2018). Repeating the measurement over an adjacent area of similar pasture cover but with zero photosynthesis, for example in response to the application of a fast-acting herbicide, provided an estimate of the evaporation component when the soil moisture condition was maintained similar to that of the active plant canopy. Using reference crop evapotranspiration (ETo) data collected from a nearby weather station yielded the evapotranspiration coefficient (Kc) and soil evaporation coefficient (Ke). The basal crop coefficient (Kcb) was then calculated following the FAO dual crop coefficient equation, i.e (Kc=Kcb+Ke) (Allen et al., 1998). Optical vegetation properties (NDVI and LAI) were also recorded from the same target pasture canopy with the help of handheld optical sensors (Green Seeker and LAI Ceptometer) to explore the relationship of evapotranspiration coefficients with NDVI and LAI.