Effective and timely monitoring of agricultural droughts can help develop an early warning system which, in turn, can minimize losses due to droughts. International organizations such as the World Meteorological Organization (WMO), the Food and Agriculture Organization (FAO), and the United Nations Environment Program (UNEP) keep a watch on the development of agricultural droughts and famines in the world. Chapters 31, 32, and 33 elaborate on operations of WMO, FAO, and UNEP, respectively. In addition, the Famine Early Warning System (FEWS; chapter 19), which is sponsored by the U.S. Agency for International Development (US-AID), monitors drought conditions in Africa. Because agricultural droughts occur due to low crop yields, monitoring them requires monitoring the factors that affect crop yields.
According to Diepen and van der Wall (1996), factors influencing yield can be categorized as (1) abiotic factors, such as soil water, soil fertility, soil texture, soil taxonomy class, and weather; (2) farm management factors, such as soil tillage, soil depth, planting density, sowing date, weeding intensity, manuring rate, crop protection against pests and diseases, harvesting techniques, postharvest loss, and degree of mechanization; (3) land development factors, such as field size, terracing, drainage, and irrigation; (4) socioeconomic factors, such as the distance to markets, population pressure, investments, costs of inputs, prices of output, education levels, skills, and infrastructure; and (5) catastrophic factors that include warfare, flooding, earthquakes, hailstorms, and frost. Measuring or estimating some of these factors is often not feasible, and the influence of some other factors may be considered insignificant or constant in an economically stable region. It is therefore weather conditions alone that affect crop yield most significantly. Various weather parameters such as temperature, precipitation, humidity, solar radiation, cloudiness, and wind velocity affect crop yield, but temperature and precipitation are most significant.
A change in temperature causes a shift in planting dates that, in turn, shifts the commencement and termination of the phenological phases. The entire crop-growth period (period from planting to harvest) may shift, shrink, or expand due to a change in temperature pattern. McKay (1983) observed for wheat production in Canada that a drop in the mean annual temperature of 1°C accompanied by a 9- to 15-day reduction in the growing season could be critical. However, interannual variation in temperature is much less than that in precipitation. It is for this reason that precipitation becomes more important than temperature for monitoring crop yields. An intricate relationship exists between precipitation and crop yield because it is the soil moisture, and not precipitation, that ultimately contributes to crop growth and crop yield.
Soil moisture data are more important than precipitation data for monitoring agricultural droughts, but soil moisture data are not as readily available as precipitation data are. Unlike precipitation data that are routinely available via a network of weather stations, soil moisture data are collected only on an experimental basis or estimated using agrometeorolog-ical models. Monitoring agricultural droughts requires soil moisture data for large areas on spatial and temporal scales. With advances in microwave remote sensing, it is becoming possible to estimate soil moisture for large areas, as described in detail in chapter 7.
Yield depends on spatial and temporal distribution of soil moisture over a crop-growth period. Soil moisture requirements by a crop vary during 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 formation in the plant, and (3) the harvesting phase, during which leaves senesce, grains harden, and the crop becomes ready for harvest. Soil moisture requirements increase rather linearly during the vegetative phase, remain 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 precipitation 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).
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 agricultural droughts are spatial and temporal anomalies in temperature, precipitation, and planting dates.
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