A Moisture-limited ET Upper Bound and Its Application in an Evaporation Pan Based Complementary-relationship Model for ET Estimation

Long-term evaporation pan observations worldwide constitute a good database to study effects of climate change. In many cases, however, actual evapotranspiration rather than pan evapotranspiration is of interest. In order to estimate land surface evapotranspiration based on pan observations, a relationship between pan evaporation and its surrounding actual evapotranspiration (ET) must be found. A complementary relationship has been developed in previous studies and is formulated as Epan = Ep + b(Ep-Ea), where Epan, Ep and Ea are pan evaporation, potential evaporation due to local radiation energy input, and actual areal evapotranspiration, respectively. We explore this model, based on long-term observations at seven evaporation pan sites in South Australia. Interestingly, we observe a clear edge in the lower-left corner of the data cloud in the (1/Ep, Epan/Ep) space for most months. This clear edge line is most likely caused by an upper bound in daily ET, which is observed at multiple AmeriFlux sites of various land covers. This upper bound in daily ET is likely associated with radiative energy partitioning and different timing of surface and root-zone soil wetting. It is simulated with physically-based numerical modelling at a vegetated semiarid environment. This upper bound in daily ET provides a mathematical basis for the lower edge in the (1/Ep, Epan/Ep) space. This edge line, when determined, can be used to estimate monthly b values. An automatic regression approach is presented to objectively determine the lower-left edge line. Preliminary calculation of Ea with the estimated b, appears to provide reasonable values for the three pan sites in areas with mean annual precipitation around and above 500 mm. We also examine under what a range of climate and surface conditions this upper ET bound exists. The significance of this phenomenon is not only in application of this evaporation pan based complementary-relationship method, but also in application of remote sensing ET methods with temporal extrapolation using the evaporative fraction, and in understanding catchment water balance and ecosystem water use efficiency. The regression-fit lower-left edge lines for estimating monthly b values at an evaporation pan site (Australia Bureau of Meteorology ID 26021). The intercept of each line is equal to b+1. The upper bound of daily ET for the month can be calculated from the slope (= -b*Ea).(The lines of positive slopes for some winter months are artefact of low resolution of pan evaporation measurements).