MODELLING OF MOISTURE UPTAKE BY PLANTS

Abstract

The present study is concerned with the prediction and field verification of the moisture extraction pattern in the crop root zone. Different root uptake models assume the moisture extraction from the root zone to follow different patterns i.e., constant, linear, non linear, logarithmic and exponential. The reason for wide applicability of the constant rate (Feddes, 1978), and linear rate (Molz and Remson, 1970; Prasad, 1988) models is that they involve parameters, which can be obtained easily in the field. Improved prediction efficiency of non-linear, exponential and logarithmic root water uptake models (Ojha and Rai, 1996, Li et al. 1999, Kang et al., 2001, Dagan and Motz, 2005) is based on the observation that the actual plant moisture extraction pattern significantly deviates from those predicted by constant and linear uptake models. These models incorporate distinctive terms representing the non-linearity of plant moisture uptake. However, crop specific non-linearity in root water uptake has not been quantified as yet. Ojha and Rai (1996) non-linear root water uptake model termed as O-R model, has versatile applicability. The model incorporates a parameter '(3', which inherits the nonlinearity of the moisture uptake pattern and effectively represents the intricacies of the moisture extraction pattern being followed by different crops. There exists an optimal value of (3 denoted as Poptimai for each crop. Parameter poptimai for variety of crops has been determined by comparing the model predicted moisture depletion with field observed moisture depletion data provided by Erie et al. (1965). The relationship amongthe individual and different combinations of parameters such as consumptive use requirement, transpiration, root depth, crop period, time of occurrence of peak transpiration/root depth, and Poptimai obtained is investigated. It is assumed that root depth is directly proportional to transpiration, and maximum root depth corresponds to the maximum transpiration (Israelson, 1962). An empirical relationship relating Poptimai to a non-dimensional parameter 'specific transpiration' has been developed. Specific transpiration involves three parameters maximum transpiration, maximum root depth and time from sowing to the occurrence of maximum transpiration/root depth. To check the field applicability of the crop specific non-linearity in root uptake pattern, Lysimeter and field crop experiments on three crops Maize, Indian mustard and Wheat are performed under controlled conditions. Various crop parameters i.e., root depth variation, leaf area index, plant height, and soil parameters i.e., soil type, bulk density, particle density, porosity, saturated hydraulic conductivity and soil moisture characteristics are determined using standard procedures. Soil moisture profile in the root zone is continuously observed at different depths using tensiometers and soil moisture measurement sensors embedded at different depths in the crop root zone and TDR soil moisture meter. The suitability of reference evapotranspiration estimation model in computing crop evapotranspiration for the local agro-climate has been investigated. Seven reference evapotranspiration estimation models, Penman Monteith, FAO-24 corrected Penman, Hargreaves-Samani, FAO-Blaney Criddle, Priestley-Taylor, Christiansen and FAO Pan- Evaporation are used to compute reference evapotranspiration from the available meteorological data. The FAO recommended crop coefficients modified for the local climatic, crop and irrigation practices are used for computing crop evapotranspiration. Actual crop evapotranspiration for different growth stages of the crops grown i.e., Maize, Indian mustard and Wheat is obtained by conducting water balance studies on Lysimeter set up. Reference evapotranspiration models based and field obtained values of stage-wise crop evapotranspiration are statistically compared. FAO-24 corrected Penman model is identified as the suitable model for local agro-climate as the computed crop evapotranspiration based on this method agree most closely with the field obtained values. The daily crop evapotranspiration is partitioned into evaporation and transpiration on basis of observed leaf area index. Field applicability of O-R model involving optimal non-linearity coefficient poptimai has been assessed by using the field observed data pertaining to Maize, Indian mustard and Wheat, and the numerical model. poptimai for each crop is estimated from the corresponding specific transpiration, based on the empirical relationship developed between the two. The field observed and model simulated moisture depletion patterns for certain durations, soil moisture profiles on number of days, and soil moisture status at different depths, in the crop period of crops grown in field i.e., Maize, Indian mustard and Wheat are compared. To ascertain the practical significance of predicting the crop specific non-linearity in moisture uptake, irrigation schedules are designed based on linear model and O-R model, at different allowable moisture depletion levels, to assess the irrigation water saving. O-R model involving poptimai coupled with soil moisture flow equation accurately predicts the soil moisture patterns in the crop root zone. Irrigation schedules based on the moisture uptake pattern predicted using poptimai based O-R model for different crops grown in the present study result in irrigation water saving. in