A biophysical model to quantify moisture stress in rain-fed rice in Sri Lanka

Typescript (photocopy).Three crop moisture stress indices (CMSI1, CMSI2 and CMSI3) are proposed. In computing index 1 and 2, stress is assumed to occur when the actual to potential transpiration ratio (AT/PT) falls below a critical level, while in computing index 3, it is assumed to occur when the daily average leaf water potential (LWP) falls below a critical level. A CMSI1 value of 1.0 is assigned to those days on which AT/PT falls below a critical value, whereas daily CSMI2 is computed as the positive difference between a critical and the actual AT/PT ratio and the daily CMSI3 is computed as the positive difference between the absolute value of a critical and the actual LWP. All three indices are weighted according to the susceptibility of the different stages of the crop to stress (Hiler et al, 1974) and the weighted daily values are summed to obtain the seasonal stress indices. Two equations are proposed, respectively, to estimate AT and AE, as a function of soil (SWP), crop and climatic parameters. The former attempts to simulate AT/PT vs. SWP curves, similar to those obtained by Denmead and Shaw (1962) while the latter is designed to simulate the two stages of evaporation (Philip, 1957). The SWP is obtained by applying a daily soil moisture budgeting procedure to a multi-layer soil model. LWP is predicted as a function of SWP and transpiration per unit leaf area, based on an experiment conducted in Sri Lanka. This experiment gave a specific resistance value of approximately 1.7E08 s for rice. The computed values of the indices and the seasonal moisture deficit suggest that prospects for rice production in the location of interest, without supplementary irrigation and/or moisture conservation measures, are limited. In this specific location, stress could be considerably mitigated by minimizing run-off losses, increasing the storage capacity of the root-zone and by the use of short-aged varieties