SEMI-ARID REGIONS
12.2 SEMI-ARID REGIONS
(i) The a.a.r. = 40 to 75 cm; a semi-arid region usually receives precipitation during winter and spring.
(ii) Rainfall occurs during a particular season of the year and it may be virtually non- existant in other parts of the year.
(iii) Only during the rainy season, the stream flow is continuous; during the dry season the stream becomes dry most of the time and carries water intermittently after an occasional heavy storm. The stream flows are usually of too short duration to develop a stable channel section and occasional high flood flows may change the cross-section radically.
(iv) Moderate precipitation and temperature higher than normal. The usual methods of determining mean precipitation by constructing Thiessen Polygons or by arithmetic mean are impracticable. Isohyetal maps should be used as the precipitation varies with time and area.
HYDROLOGY
Average elevation-precipitation curves from precipitation data will be more useful for semi- arid regions.
...(12.2) where R = annual rainfall in cm T = annual average temperature in °C (v) Major loss of water is by evapotranspiration and the evapotranspiration losses range
R lies between (T + 14) and 2 (T + 14)
from 40 cm/year for barren rocky areas to 90 cm/year for heavy forest areas. (vi) The ground water table occurs at moderate depth. (vii) Wide variability of hydrologic factors with respect to time and space are a charac-
teristic feature; for example, vegetative cover may vary radically across a basin–thick vegeta- tion in higher altitudes and barren land in lower reaches.
A high elevation zone of a river basin may get precipitation regularly while other parts of the basin may not get; 90% of runoff may result from 10% of the drainage area. Runoff
values expressed as flow per km 2 of drainage area may be inapplicable.
Hydrologic factors occurring in one season may be virtually non-existant during an- other season.
As geology of the basin differs from place to place, the infiltration rate too differs. (viii) Seasonal crops like jowar, ragi etc. can be grown. (ix) Probability and duration studies (flow duration, drought duration) are essential to
determine the available flow or drought period in a long term climatalogical cycle (or swing). (x) A drought occurs at least once in a year except in abnormal years, (xi) Even in a single river basin, the hydrological processes in semi-arid regions differ
from area to area. (xii) Mean depth of precipitation. In semi-arid regions where the precipitation is controlled
by the topography, the pattern of rainfall distribution can be studied by ‘isopercentral map’. Such maps should be prepared for individual and isolated storms. By adopting isoper-central method, a comparatively small number of stations can be used to develop a quite detailed isohyetal map. This method of finding the mean precipitation over the basin will avoid errors caused by imperfect distribution of raingauge stations.
Because of the characteristic variability of precipitation in semi-aird mountainous re- gions, direct transposition of either isohyetal pattern or depth-area values from one river basin to another may lead to quite unreasonable results.
(xiii) The unit hydrograph technique can not be used in semi-arid regions, as it requires constancy of hydrologic factors over a river basin. (xiv) Semi-arid and desert regions of Rajasthan. Rajasthan (n-w India) has a total area of ≈ 330000 km 2 , which is sandy and unproductive with scanty rainfall and very low water table. About 90% of the rainfall occurs during the monsoon period, June to September. The evapora- tion is very high of the order of 300 cm and much of the rainfall is quickly evaporated, though some portion of it sinks into the earth to replenish the ground water.
The conditions in the semi-arid regions are better than the desert lands. Some portion, particularly the Luni basin, can probably be improved if suitable conservation, storage and irrigation methods are adopted. For planning the reclamation of the desert and semi-desert regions of Rajasthan, it is necessary to have proper study of meteorological conditions based on detailed observations of rainfall, evaporation, humidity and other meteorological elements.
ARID, SEMI-ARID AND HUMID REGIONS
(xv) Silting. While sheet erosion is dominant in regions of moderate rainfall, gullying and stream channel erosion are characteristic of arid and semi-arid regions. Sediment depos- its in the upper end of reservoirs generally become covered by vegetation resulting in heavy evapotranspiration loss of the available water, which is more critical in arid regions.