|Design and Operation of Smallholder Irrigation in South Asia (WB, 1995, 134 p.)|
|Chapter 4 - Water supply and demand|
The region under discussion is monsoonal, with river-flows characterized by highly variable seasonal and annual discharge. Storage reservoirs can be provided in some cases where topographic and ecological considerations permit. In general storage capacity can provide partial regulation only, and the project must accommodate to such limited control of flow, or to completely unregulated flows in the absence of any reservoir.
The highly variable nature of monsoon precipitation makes for considerable difficulty in both yield and flood hydrology. The onset of the monsoon, upon which so many agricultural operations depend, may vary by several weeks from year to year, and gaps of weeks duration may occur within the monsoon period. Much of the monsoon precipitation is in the form of discrete, local rainstorms, often violent, rather than the popularly conceived uniform countrywide downpour. This pattern results in wide random variations in seasonal rainfall between adjacent areas (as much as 50% difference in a particular year, between locations as little as 25 km apart) and makes for considerable difficulty in a statistical approach to estimation of water yield, particularly for small catchments. The problem is aggravated by the limited number of rainfall and river-flow recording stations. While international agencies commonly call for at least five years of actual stream-flow records as a basis for the design of small projects, (much longer for major projects) in remote areas there are commonly none and extrapolation from similar catchments must be resorted to. In these circumstances expansion of the network of rainfall recording and stream gauging stations is a priority item. It is noted, however, that maintenance of calibration of stream gauging stations is no small task in rivers subject to heavy siltation and frequent changes of channel during flood-flows.
The impact of the widely varying pattern of monsoon precipitation on the life of the small cultivator is illustrated by two situations. In one, the monsoon had begun propitiously and then failed, and paddy stood wilting in the fields. It was ploughed in, an unusual event, and when the rains returned was replanted with yellowing spindly seedlings remaining from seed-beds. The monsoon then became violent, flooding and destroying the replanted crop. Cultivators in the area, in the path of monsoon storms moving from the Indian ocean to the Himalaya, commonly borrow ostensibly for purchase of fertilizer but actually for "pujas", religious ceremonies to placate the deity held to be responsible for such outrageous events.
In the other case, the young maize crop, newly sprouted from the red lateritic soil, stood wilting under the backdrop of heavy grey monsoon clouds, but it did not rain. And nearby, the Door of the village reservoir was cracked and dry. The monsoon had failed for two successive years. The next monsoon rains were nine months away.
The seasonal variations in monsoon rainfall can, of course, be studied statistically, and this must be done in project design, but the realities of the situation for the cultivator and his family must also be kept in view.
Given the large variability in water supply, the immediate problem is to take into consideration the uncertainty of water supply into the design of the project. To design for an assured level of supply would avoid certain operational problems, but would grossly underutilize the water available.
Much of the debate over the design and operation of surface irrigation systems centers around the question of how to handle the non-assured component of supply. One approach to limiting the variability of supply to be accommodated is to design the system for the "75% probable" year (or other degree of probability). Then statistically in three years out of four, the amount of water available equals or exceeds the amount for which the system is designed; only in the fourth year is there a deficit. A calendar of twelve months each which is "75% probable" may also be constructed, becoming the "design years". While this is a useful concept for purposes of establishing system capacity, it still leaves the question of how to operate the system in the deficit years, or months. This will be discussed in the next chapter.
If the system is to have storage, a question influencing design and operation is how the storage will be utilized, whether for seasonal regulation within a twelve-month period, or over-yearly. In the first case water stored in the wet season is used in the following dry season, possibly with some carry-over for pre-monsoonal irrigation (particularly puddling and transplanting of paddy) in the following year. In the second case, applicable only to major reservoirs, the storage cycle may extend over several years, partially evening out years with good and bad water supply.