Adaptation of conventional evapotranspiration models to estimate irrigated apple tree transpiration

Evapotranspiration is driven by the meteorological conditions but it is also dependent on factors imposed by the plants and by the amount of soil water available to the roots. The problem of determining water use by plants is very complex and involves the need of measuring many environmental and biological variables. Penman (1948) showed that part of the energy used in the transpiration comes from the radiation balance, and another part is supplemented by the sensible heat from the atmosphere which is converted into latent heat.

When a large portion of land is continuously covered by an aerodynamically fairly smooth vegetation, the radiation balance is likely to be the predominant source of energy for the transpiration, and Priestley and Taylor (1972) have found that it accounts for 65% to 75% of the total transpiration. If the vegetation is fairly rough and sparse, then the atmospheric sensible heat input can contribute with 60% to 70% of the total (Thorpe, 1978; Green, 1993).

Many models developed for the first situation have been adapted to estimate transpiration of orchard and isolated trees. Such adaptations need auxiliaries sub-models and depend heavily on many site specific data and calibrations with empirical coefficients.

The approach proposed here is very simple and intended to be universal. It takes the estimates from conventional grass reference evapotranspiration and correct them by a factor given by the ratio between the tree leaf area and the grass leaf area where the net radiation is measured. Four methods were tested: Penman, Penman-Monteith, Priestley-Taylor, and the Morton's approach to Bouchet's complementary model.

Actual apple tree transpiration obtained through sap flow measurements in drip irrigated orchards were used to test the proposal. The results obtained with 14 trees, with leaf area ranging from 4 m^2 to over 20 m^2, in two different experiments, during two independent years, for different cultivars and environmental conditions gave surprisingly good fit and are encouraging. Now, the major problem is to find an easy way to estimate tree leaf area.