Remote Sensing assessment of water stress effects on wheat
R. K. Mahey, Rajwant Singh
S. S. Sidhu, R. S. Narang
Department of Agronomy Punjab Agricultural University
Ludhiana-141 004, India
Abstract
In order to monitor vegetation conditions, remote sensing techniques have been used successfully for several crop covers. The objectives of the study was to investigate the potential usefulness of spectral measurements to estimate leaf area index, biomass and detecting water stress in wheat (Triticum aestivum L.) Ground-based radiometric measurements were made on seven irrigation treatments throughout a complete crop cycle in order to monitor wheat growth and development under irrigated and stressed conditions in an experimental field at Punjab Agricultural University, Ludhiana during winter season of 1987-88 and 1988-89. Reflectance in the red (625 to 689 nm) and infrared (760 to 897 nm) bands was measured with hand held radiometer. Concomitant measurements of some of the agronomic variables were also made. Canopy air temperature difference(
DT) was recorded at maximum crop cover stage. Spectral data have been correlate with plant height, leaf-area index, total wet/dry biomass, plant water content, grain yield, consumptive use of the crop and canopy air temperature difference. The results shows a significant correlation between spectral data derived from near infrared, red radiances and various agronomic variables. Infrared: red reflectance ratio (R) and normalized difference (ND) vegetation index were found highly and linearly correlated with yield establishing the potential of remote sensing for predicting grain yields. The correlation for R and ND was maximum during 75-104 and 76-102 days after sowing respectively during the two seasons.
DT was also significantly related to yield as well as spectral parameters. The different temporal spectral response under no irrigation treatments also showed the usefulness of spectral measurements in detecting water stress effects on crop. So the results of the experiments show conclusively that a hand held radiometer can be used to collect spectral data which can supply information on whet growth, development and detecting water stress effects.
Introduction
Water is an important input for crop productivity which varies from place to place. Crops suffer from water deficit and its yields are reduced. Water availability will remain an important factor in years t come, thus an assessment of crop response to water availability under field conditions and knowledge of it becomes an essential requirements. From remote sensing devices operated from airborne system or satellites, it may be possible to make a quick assessment of vast areas. However, in the present development of remote sensing technology in India, an understanding of plant response to water deficit which can be recorded by remote sensing devices is a basic requirement in this direction hand held radio meters can be used to develop fundamental data on ration between radiometric data and crop growth parameters. Keeping in view the above considerations, the present field experiments were conducted on wheat.
Materials and Methods
Field experiments were carried out on wheat (Triticum aestivum L. var. WL 711) at Punjab Agricultural University, Ludhiana during Rabi seasons of 1987-88 and 1988-89. The field was irrigated and ploughed and wheat was sown and harvested in second week of November and April respectively in both the years. The experiment consisted of seven irrigation treatments, viz. no irrigation; one irrigation at crown root initiation (CRI); two irrigations at CRI + tillering (T)/Flowering (F); three irrigations at CRI + F + milking (M); four irrigations (as per recommended practice of 21, 62, 100, 125 days after sowing); five irrigations at CRI + T + booting + F + M; and irrigations based on cumulative pan evaporation of 75mm. In the last treatments total 3 irrigations were given which were equivalent to CRI + F + M stages.
The indigenously developed hand held radio meter was used to measure the radiance in situ from these plots on 8 measurement dates. The radiance was measured in red (625-689 nm with a peak at 665nm) and infrared (760-897nm with a peak at 830 nm) normal to the ground surface at a height of approximately 1.5 m above the crop canopy. Four to six spectral measurements per plot were averaged to account for the spatial variability of each plot. Immediately after each spectral measurement on a given plot, solar irradiance was measured fro a BaSO
4 panel. All these measurements were normalized with the irradiance obtained by the BaSO
4 panel. To describe growth patter radiance ratio ® of IR/Red ad normalized difference (ND) i.e. (IR-red/IR+red) were used on spectral parameters.
Results and Discussion
- Spectral Response vis-à-vis Plant Growth
Parameters
A simple linear (Model I) ad quadratic (Model II) regression analysis was used to related R and ND with growth and agronomic variables during 1987-88 and 1988-89. The relationship of parameters Viz, plant height, total fresh biomass, total dry biomass with R and ND was improved when model II was used (table 1). However, the plant height and the leaf-area index were linearly correlated with spectral indices (Fig. 1c). It was observed that there was general increase in R and ND with an increase in agronomic values during the vegetative growth but the trend was reserved in the later crop states. It appears that R and ND are function of the agronomic growth parameters.
Table 1: Correlation coefficients resulting from regression analysis during 1987-88.
| Agronomic variable |
Spectral parameter |
| Infrared: red |
ND |
| Model I |
Model II |
Model I |
Model II |
| Plant height |
0.13 |
0.94 |
0.26 |
0.91 |
| Leaf-area index |
0.59 |
0.62 |
0.41 |
0.48 |
| Plant water content |
0.23 |
0.27 |
0.24 |
0.26 |
| Total fresh biomass |
0.14 |
0.63 |
0.28 |
0.73 |
| Total dry matter |
0.23 |
0.58 |
0.34 |
0.55 |
- Crop development
The growth and development of crops can also be represented by the temporal variation of spectral parameters over the crop cycle. Radiance ratio and ND increase in the beginning with increasing green biomass, becomes maximum and then decreases due to senescence. The IR/red ratio and ND was always higher for an irrigated crop compared with unirrigated one (Fig. 1a&b). The difference in these spectral parameters for irrigated and water stressed whet were more during 75 to 102 days after sowing. Ti was found that at 85 to 91 DAS unrrigated and normal (4 irrigations) irrigated wheat differ significantly from one another with respect to radiance ratio and ND in both the years. Thus, spectral discriminability in irrigated and stressed plants is enhanced during this period. Similar result have been found by Kamat et. al. (1985).
