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Agriculture/Soil
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Use of Remote Sensing technique in scheduling Irrigation
D.K. Das
Division of Agricultural Physics
Indian Agricultural Research Institute
New Delhi-110 012, India
Abstract
Increased interest in the utility of Remote Sensing technique for efficient water management has necessitated study of the possibility for its use in irrigation scheduling for crops over large areas. The paper discusses the different remotely sensed canopy temperature derived indices and their implications in scheduling irrigations for different agricultural crops. Research results based on experiments conducted in semi-arid region of Delhi, India are presented to show that in wheat irrigation based on standard deviation of midday canopy temperature (CT-SD) of ±-10.30°C produced yields of 98.30 and 97.00 q ha-1 of dry matter, 42.92 and 41.62 q ha of grains with a saving of 100 and 60mm of irrigation water over the plots frequently irrigated at critical growth stages. Irrigation based on canopy temperature derived indices also resulted in lowering of leaf diffusion resistance (LDR) and was associated with an increase in leaf water potential (LWP), transpiration rate and lowering of canopy temperature over the water stressed wheat crop thus maintaining favourable canopy environment in the former. The results demonstrated the potentiality of irrigation scheduling based on remotely sensed canopy temperature to attain higher yield and water use efficiency of crops.
Introduction
Efficient water management is key to success in augmenting crop production. In developing countries such as in India, which has created the highest irrigation potential in the world, the use efficiency of irrigation water resource is quite low ranging from 20 to 40 percent owing mainly to improper irrigation schedules, over irrigation, excessive seepage and percolation. The conventional methods of scheduling irrigation based on either per cent soil water depletion, irrigation water-pan evaporation ratio (IW/CPE) or crop growth stages often result in greater number and amount of irrigation than actually required for specific crops. Under conditions of low evaporative demand, water extraction by a plant may balance the transpiration demand over a relatively greater depletion of soil moisture but under high evaporative demand, even a high soil moisture level may result in yield reduction level crop water deficit. Moreover the point measurements have limited application have over large areas. Hence increase in irrigation water use efficiency necessitates improved irrigation scheduling techniques based on integrated effects of climate-soil-crop characteristics.
In recent times there has been increasing interest in the use of newly developed Remote Sensing techniques for efficient management of economic and costly water resources (Sahai, 1990; Da et al. 1990). The use of crop canopy temperature measured through remotely sensed sensors (by satellite, air-born or ground based sensors) had opened up new vistas for control of crop water supply, in proper scheduling of irrigation and better utilization of water resources (Jackson et al., 1977; IDSO et al., 1977; Idso et al., 1977; Das and Kalra, 1990). This report briefly discusses the thermal indices for detection and quantification of plant water stress, some research results concerning the use of these indices in scheduling irrigation and the implications in adopting the frontier technology for farm water management.
Canopy Temperature Based Indices
Thermal-IR techniques can be used to detect and quantify plant water stress.
The methods are associated with increased leaf temperature variability in a cropped field with restricted transpiration because of a deficit in water supply (Tanner, 1963). Four canopy temperature based indices have been developed for detecting plant water stress and scheduling irrigation; (i) canopy-air temperature difference (CATD) and stress degree days (SDD), (ii) canopy temperature variability (CTV), (iii) temperature stress day (TSD) and (iv) crop water stress index (CWSI) (Jackson et al. 1986).
- Stress degree day is the cumulative difference between the canopy temperature (T) and air temperature (T) measured post-noon near the time of maximum heating (Idso et al., 1977; Jackson et al., 1977). It is assumed that the canopy temperatures would account for the effect of environmental factors such as vapour pressure, net radiation and wind. The SDD increases with increasing plant water stress (Fig. 1, 2). A crop is considered stressed if the value is high and positive and unstressed if it is negative. This change over is, however, arbitrary and may not be valid for all environments.
- The canopy temperature validity (CTV) is the variability of temperatures encountered in a field during a particular measurement period. It is expressed as the standard deviation of mid-day canopy temperature within a field. The basis for CTV index is that soils are inherently non-homogeneous. Some areas within the field becomes stressed earlier than others. As water limiting in the former, the canopy temperature would show a greater variability. This variability can be used to signal the onset of deficit and schedule irrigation (Gardner et al., 1981)
- The temperature stress day (TSD) is the difference in temperature between a stressed plot and a well irrigated plot (Gardner et al., 1981). Use of well watered plot as reference compensates for environmental effects. It needs to be in the vicinity of the field to be irrigated.
- The Crop water stress index (CWSI), defined as CWSI= (1 – AET/PET) is generally accepted as a global quantitative assessment of the water stress (Jackson, 1982). AET is the actual evapotranspiration (Water effectively used as a daily or large time scale equivalent of LE) and PET is the estimate of potential or maximum evapotranspiration. By using the steady state energy balance for a crop canopy CWSI was computed for irrigation scheduling purposes based on relationships between canopy air temperature differences and vapor pressure deficit (Idso et al., 1981; Jackson et al., 1981). It is computed as the ratio of the observed (Tc – Ta) for the given condition of air saturated deficit e (in mb or Kp) at the time of observation to the maximum possible (Tc – Ta)max for a fully dry crop in the same conditions (Fig. 3).
Testing The Indices for Scheduling Irrigation
CATD and SDD: Ehrler (1973) suggested that leaf-air temperature differences could be used as a guide irrigation scheduling. In field experiment on wheat, profile water depletion had been correlated with SDD (fig. 2) and it had been suggested that irrigation to whet might be applied when the positive SDD value was 10 or below (Jackson, 1977). Scheduling irrigation for snap beans based on SDD resulted in similar grain yield to that of irrigation based on soil water potentials or growth stage (Bonano and Mack, 1983). Geiser et al. (1982) developed an irrigation scheduling model in corn using CATD on the dependable variable and net radiation, relative humidity and available soil water as independent variables. The water balance and resistance methods called for additional water application of 39 and 18 per cent, respectively when compared with temperature difference method.
CTV : The average canopy temperatures of a water stressed plot might no typify water stress. On the other hand the canopy temperature variability indicated areas of adequate or inadequate water in a field. A measure of temperature variability, therefore, might be used in irrigation management (Nixon et al., 1973). Fully irrigated plots of corn exhibited standard deviation of CT ± 0.3°C whereas in non-irrigated plots the S.D. was as high as ± 4.2°C. It was, therefore, concluded (Gardner et al., 1981) that crops exhibiting S.D. above ± 0.3°C were in need of irrigation.
TSD : In testing the concept of temperature stress days (TSD) for irrigation scheduling, irrigation was applied to corn when the average of all CT measured in stressed plot during a time period 1°C warmer than the well irrigated plot (Calwson
and Blad, 1982). Comparison of CTV and TSD indicated that the former be used to signal onset of plant stress but the severity of stress was better indicated by the magnitude of the elevation in average CT above that of a well watered reference plot.
CWSI : Computation of crop water stress index based on CATD and vapour pressure deficit, indicated that with CWSI of 0.3, a reduction in growth rate was imminent. If it reaches 0.5, net growth would cease and might decrease. Irrigation should, therefore, be applied when CWSI is between 0.3 and 0.5 (Jackson, 1983). Originally the approach was developed and verified under warm and dry climatic conditions, and only limited work (Keener and Kirchner, 1983; Pennington and Heathery, 1989) has been conducted using the approach under humid conditions.
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