SOIL MOISTURE UPTAKE BY PLANTS UNDER SALINE IRRIGATION

Abstract

The present study is concerned with the prediction and field verification of moisture extraction pattern in the crop root zone under the saline and non-saline conditions. In general, root water uptake is acomplex phenomenon in nature because it depends on various factors such as soil parameters, crop parameters and atmospheric conditions. There is an existence of non-linearity in the root water uptake in the crop root zone. Various root water uptake models are available which follow different root water uptake pattern i.e. constant (Feddes et al. 1978), linear (Prasad, 1988), non-linear (Ojha and Rai, 1996) and exponential (Li et al. 1999). Of which crop specific nonlinearity is dealt by Ojha and Rai (1996) model and represented as O-R model. The non-linearity in the root water uptake is presented by O-R model, in which model parameter p which inherits the non linearity of the moisture uptake pattern. In the present study numerical model is developed to simulate the one-dimensional vertical flow ofmoisture through unsaturated zone (Richards equation) using fully implicit finite difference technique with the sink term given by non-linear O-R model. The solution ofthe model is obtained by adopting different boundary conditions. The input parameters required for the model simulation has been obtained from the experimental studies conducted at Field Lab of hydraulics section, I.I.T. Roorkee, Roorkee. A field selected for conducting the experiments is divided into 4 plots. Plots 2 & 3 are irrigated with fresh water and plots 1&4are irrigated with two different levels ofsalinity (4dS/m and 6.25dS/m) in fresh water. Two major crops of India, paddy and wheat were used for cultivation. The experimental work includes collection of meteorological data, soil moisture measurement, soil parameters (soil texture, bulk density, saturated hydraulic conductivity field capacity and permanent wilting point), plant parameters (leaf area index, crop height, root depth, daily crop evapotranspiration), salinity (electrical conductivity ofthe soil sample), and soil quality parameters such as chlorides, nitrates, sodium and calcium. Salt balance study is done for two levels of salinity for both the crops. During the initial stages ofpaddy crop growth, salt content is less and increases during crop development stage and reduced during late season. Similarly for wheat, salt content accumulation is less at initial stages and increases as the crop growth stage proceeds and again reduced in the late season. In both the saline plots, as the crop growth stages proceed, the capability of the crop growth increases. Because of salinity, the hydraulic conductivity can be affected and is assessed by conducting experiments. Experiments were performed in the laboratory for field soil samples (saline as well as non-saline) collected at ground surface and in 30cm depth for finding the hydraulic conductivity versus moisture content relationship. An exponential form of equation is established for the hydraulic conductivity for two saline treatment levels and also for fresh water sample. The developed numerical model is used to simulate soil moisture depletion and soil moisture profile under two cases, a). Irrigation with fresh water b). Irrigation with saline water (at two different levels of salinity) for the crops paddy and wheat. Optimal value of p has been obtained for case(a). The model simulations are in good agreement with the field observed data. It is observed that moisture depletion at the top layer ofthe root zone is higher for paddy compared to wheat. For saline case, optimal p is evaluated by adopting two strategies. In the first strategy, the role of using a correction factor (C.F) in crop coefficient is evaluated and is observed that, the approach proved to be insignificant for simulating percentage soil moisture depletion. In the second strategy, C.F obtained for hydraulic conductivity from the experimental observations is used in the modeling. The optimal value of P and the error estimate, Co efficient of Determination (COD) is estimated for the modified hydraulic conductivity. Optimal p and C.F is varied for both the saline cases based on the COD values. The best matching pair of optimal pand C.F is obtained by drawing contours for p and C.F corresponding to maximum COD. Forthe best matching pair, percentage soil moisture depletion and soil moisture profile are plotted for certain durations for both paddy and wheat. Use of correction factor to hydraulic conductivity improves simulation in case of saline soils as the results obtained between field observed and model predicted moisture content are in better agreement. The non-linearity parameter p is found not to differ more than 10% in case of saline soils. 111