Vergara - 1976 - Physiological and morphological adaptability of ri

CROP PLANNING into RAINFALL 59
value chosen for any crop under consideration. Different criteria can be used for
different stations. depending on soil characteristics. The results may then be
integrated over the entire area.
Thus, depending on the nature of the problem, ajudicious choice of threshold
or limiting values of the amount of rainfall and the duration of dry or wet spells
for analyzing rainfall data could provide information useful in crop planning.
This is not to suggest that other climatic parameters should not be considered.
Where extensive data on various climatic elements are available, each parameter
may be analyzed. using more or less sophisticated methods. to derive quantitative
results. This aspect is further discussed below.
CLIMATIC CLASSIFICATION AND CROP PLANNING
Crop planning in relation to climate usually begins with an attempt to arritte at a
“climatic” or an '“agrocli1natic" classification of an area. A number of criteria
such as ratio of precipitation and temperature. moisture index. length of growing
season. vapor-pressure deficit. etc. have been proposed and are utidely used
(eg. Koppen, 1936: Prescott. 1938; Thornthwaite. 1948'. Papadakis. 1965; Thran
and Broekhuizen, 1965). The practical usefulness as well as the limitations
of these classifications with special reference to agricultural planning have been
adequately reviewed by Burgos (1968) and de Villiers (1968).
Methods based on soil-uxater balance models (cg. Butler and Prescott, 1955;
Fuchs and Stanhill, 1963; Wallen. 1968; Cocheme. 1968) require estimation of
potential and actual cvapotranspiration, deep drainage, and soil “tater availability’.
and involve a number of simplifying assumptions. The measurement
problems associated with the evaluation of the soil water balance have been
enumerated by Slatyer (1968a). and further assumptions may have to be made
when these models are to be extended for regional crop planning (Slatyer.
1968b). However these classifications are indicative of the agricultural potentialities
of the different regions and provide broad specifications for the plant
breeder in the development ofcrop varieties suitable for the area under consideration.
While crop characteristics could be manipulated to a certain extent by
genetical improvement. our ability to modify climate under field conditions is
limited thus far.
Crop planning and varietal characteristics
One of the main objectives of crop planning is to identify the areas where a
particular crop could be grown successfully under given climatic conditions.
Nearly every crop variety responds in its town way to the atmospheric environment,
and the same climatic element can have quite different effects even on a
single variety. depending on its stage of growth. Varieties also may differ in their
ability to ivithstand drought at different stages of growth. For example. rice
varieties like Cauvery wilt at moderate drought but rejuvenate better than most
other varieties after re-irrigation (Balakrishnarao ct a1, 1971).