ABSTRACT
Abstract Surface temperature measured with infrared thermometers is an important tool for irrigation scheduling that has been in practice for some decades Several indices have been developed to time irrigation events The most useful is the Crop Water Stress Index (CWS1). Its use however relies in a non-water-stressed baseline that up to now, although having a theoretical basis, is to be determined experimentally given the uncertainties on the surface resistance of the crop Drawbacks of this procedure, besides the non-transferability of the lines from local to local, are that the surface temperature measurements are to be made always at the same time of the day and under similar weather conditions. A new definition of a non-water stressed baseline theoretically based and driven by weather variables that can be easily measured and/or estimated is proposed that allows measurements at any time of the day, thus simplifying the task of the irrigator Keywords: Crop water stress index, energy balance, evapotranspiration, infrared surface temperature, irrigation scheduling, vapour pressure deficit
1 Introduction
One of the main decisions an irrigator is faced with is the timing of irrigations Several methods exist either plant, soil or atmosphere based, that may be more or less time consuming or that may rely on expensive equipment
The arrival of commercial portable infrared thermometers for surface temperature measurements represented a great advance in this area. They are used mainly to detect crop water stress, and several indices have been proposed as an aid to irrigation
Water and the Environment: Innovative Issues.in Irrigation and Drainage. Edited by Luis S. Pereira and John W. Cowing. Published in 1998 by E & FN Spon. ISBN 0 419 23710 0
scheduling, like Stress Degree Day (SDD), Critical Temperature Variability (CTV), Temperature Stress Day (TSD), and, specially, Crop Water Stress Index (CWSI) [1]
The CWSI method relies on two baselines: the non-water-stressed baseline, that represents a full watered crop, and was first introduced by Idso et al. [2], and the stressed baseline, that corresponds to a non-transpiring crop (stomata fully closed). Both lines are drawn on a Ts-Tn vs. VPD plot (Fig. 1), where 7 .^ and Ta are, respectively, the surface and the air temperature at the reference level (°C) and VPD is vapour pressure deficit (Pa)
Fig. 1. Baselines used for the computation of CWSI (schematic representation).
