Agricultural Drought and Its Monitoring and Prediction: Some Concepts - Monitoring and Predicting Agricultural Drought: A Global Study - page 6

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C rop Yield and Soil Moisture

Yield depends on spatial and temporal distribution of soil moisture over
a crop-growth period. Soil moisture requirements by a crop vary dur-
ing this period, which consists of three main phenological phases that
develop in sequence: (1) the vegetative phase, which includes the period
from planting to the complete leaf (or canopy) development, (2) the grain-
filling/heading/reproductive phase, which includes the period of grain for-
mation in the plant, and (3) the harvesting phase, during which leaves
senesce, grains harden, and the crop becomes ready for harvest. Soil mois-
ture requirements increase rather linearly during the vegetative phase, re-
main at the peak during the reproductive phase, and decline during the
harvesting phase. Moisture deficiency during the reproductive phase affects
crop yield most significantly (Mahalakshmi et al., 1988).
A crop needs an adequate amount of soil moisture on a continuous basis
throughout the growth period. Irrigation can meet such needs. However,
in the absence of irrigation facilities, crop growth relies on precipitation.
The amount as well as the temporal and spatial distribution of precipi-
tation influence crop yields. Lower but well-distributed precipitation may
result in a higher crop yield as opposed to higher but poorly distributed
precipitation. The variation in precipitation is one of the significant factors
causing agricultural droughts (Liverman, 1990).

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C rop Yield and Planting Dates

[6],

Planting dates significantly affect crop yield and probability of agricultural
droughts (Mahalakshmi et al., 1988; Kumar, 1998). Each crop has an ideal
time window for its planting. A crop planted early or late may not reach
its potential yield. Chapter 2 describes physiological characteristics of some
major food crops (e.g., wheat, rice, maize, and millet). An understanding
of these characteristics is essential to studying the impact of precipitation
on crop yield. The major factors that contribute to the occurrence of agri-
cultural droughts are spatial and temporal anomalies in temperature, pre-
cipitation, and planting dates.

U se of Satellite Data
Weather data are commonly used to estimate crop yield and agricultural
droughts. However, weather data are collected only on point locations and
do not adequately represent the spatial coverage by crops. In contrast,
satellite data have greater capability to monitor crop condition in a spa-
tially continuous fashion and on a regular time interval and therefore have
proven to be a dependable source for monitoring crop yields and agricul-
tural droughts. Various types of satellite data are currently available for
monitoring crop conditions. These can be categorized into: (1) visible and
infrared data, (2) passive microwave data, and (3) active microwave data.