Canopy resistance modelling for crops in contrasting water conditions

Abstract

Although canopy resistance to vapour water transport (r c) depends on climatic conditions and crop water status, standard constant daily values are usually used. Thus models using r c to predict evapotranspiration (ET) fail if applied to water stressed crops. On the other hand, in the scientific literature it is possible to find daily r c models dependent on soil moisture, but, in such cases, these need to be calibrated for each crop and site. Here a “climatic resistance” (r∗) is introduced as function of available energy, vapour pressure deficit and air temperature. Therefore a model of canopy resistance is presented on a hourly and daily time scale, where r c is expressed as function of r∗, aerodynamic resistance, r a, and predawn leaf water potential (PLWP), independently on the soil type. The model has been tested in Southern Italy on grass (reference crop), sorghum, sunflower and soybean and validated in France on soybean, without further calibration. The field crops were submitted to several water stress cycles: PLWP ranged between −0.1 and −1.2 MPa. The experiments showed that this model works well both under and without soil water constraints. On an hourly scale calculated ET in function of PLWP always presented a small underestimation (maximum 6% for soybean in Italy under senescence and water stress); on a daily scale these underestimations are reduced in general. The model test showed that it is independent of the site but depends only on the crop species. On a daily scale the model is presented also with available water (AW) as input, but in this case it needs local calibration. When AW is used as input the model showed an underestimation of 5% and 7% for sorghum and sunflower respectively.