Development of a plant water stress index based on soil moisture supply and atmospheric evaporative demand for agricultural irrigation management

Abstract:

Soil moisture deficit and atmospheric aridity (high vapor pressure deficit, VPD) both can cause reduction of agroecosystem productivity. Traditionally, agricultural irrigation management have primarily focused on soil moisture deficit (plant water supply) to quantify plant water stress, but largely neglected plant water demand from atmospheric aridity. We argue that because plant water stress is co-limited by soil moisture supply and atmospheric evaporative demand, a plant-centric plant water stress index should be defined holistically based on the interplay between soil moisture supply, atmospheric evaporative demand, and plant physiological regulations (i.e. plant hydraulics and stomatal response) for agricultural irrigation management. Empirical evidence from greenhouse experiments and eddy covariance measurements shows that stomatal conductance is co-regulated by soil moisture and VPD from the supply and demand aspects. Plants can have water stress even with high soil moisture but under high VPD; while plants may not have water stress when soil moisture is relatively low and VPD also happens to be low. Thus, a new plant water stress index (PWS) based on stomatal conductance is proposed to indicate plant water shortage for irrigation scheduling. The dynamic irrigation triggering threshold is determined based on the new plant-centric PWS index. We used an advanced process-based model, Ecosys, to reproduce the co-limitation of water supply and demand on stomatal conductance. We then used the Ecosys model to implement the new plant-centric PWS index and the traditional soil moisture-based index (i.e. maximum allowable depletion, MAD) for agricultural irrigation management at 12 sites with a rainfall gradient across Nebraska. We found that compared with the soil moisture-based irrigation rule, the PWS-based irrigation rule can significantly reduce irrigation water use (-22.6%), maintain crop yield, and increase economic profits (+20.4%, $11.2/acre). Our results demonstrate that the proposed plant-centric PWS index can effectively advance agricultural irrigation management.