|Design and Operation of Smallholder Irrigation in South Asia (WB, 1995, 134 p.)|
|Chapter 4 - Water supply and demand|
Rice is the most important single crop in the region under discussion. It is the only food crop which can be grown under conditions of continuous inundation of the root-zone, a feature which makes it uniquely suited to wet-tropic monsoonal cultivation. However yields are also responsive to sunshine, and are inhibited by cloud cover. Hence, highest yields are obtained in lower rainfall areas under irrigation as in the Punjab.
Rice is conventionally grown under conditions of inundation, when it is referred to as paddy (the term is also used for the bunded plot in which rice is grown) or as wet-land or low-land rice. It can also be grown without inundation, soil-moisture being held at near field capacity, in which case it is generally referred to as upland rice. It is basically the same plant in either case, although preferred varieties for the two situations may differ. Between the two limits, of continuous inundation on the one hand and upland cultivation on the other, lies a wide range of conditions under which rice can be successfully grown and which have a considerable bearing on water requirements.
For wet-land paddy, water is required for cultivation and puddling, and to compensate for seepage and to meet evapotranspiration. Cultivation (initial plowing) may be carried out in dry conditions, but in view of the limited capacity of the draft animals employed prior softening of the soil either by irrigation or by pre-monsoon showers is desirable. Subsequent puddling serves to convert the soil into a fine saturated slurry suitable for transplanting. It also provides weed control, and reduces seepage rate.
With regard to estimation of water requirements for cultivation and puddling, there are two widely different approaches. In traditional wet-tropic areas cultivation and puddling of a plot may extend over a period of a month or more. Emphasis is laid on the merits of allowing time for rotting of the ploughed-in stubble of the previous year's crop, under saturated conditions, before completion of puddling and transplanting the new crop in order to conserve nutrients. In contrast, there are extensive areas where water requirements and time are critical, where cultivation, puddling, and transplanting of an individual plot all occur within a period of twenty-four hours. The difference in water requirements between the two procedures is, of course, substantial (300 to 400 mm compared with 150 mm). It is noted, however, that even where puddling and transplanting in each plot is carried out in short order the operation is likely to be in progress in a large command over a period of several weeks due to limitations in availability of labor, draft animals, and cultivation equipment.
The capacity required of main and distributary canals during puddling in an area predominantly under paddy in the wet season is influenced both by the amount of water used per unit of area (the procedure employed on the individual plot), by the amount of time during which this operation is in progress in the command as a whole, and by the contribution of rainfall during the period. It should be noted that a plot puddled and transplanted early is likely to have little assistance from rainfall during the process, while a plot prepared later may benefit from already being saturated from prior rains. Consequently, averaging water requirements over the whole command is not entirely appropriate to determine the rate of supply required (the "water duty") for an individual sub-area. In this regard, the practice in some small village schemes is to make cultivation, puddling, and transplanting a communal or social event, with the whole population of the village concentrating its labors in one local area at a time, with virtually the entire flow of the main canal temporarily directed into that area. The water duty required at the tertiary or minor canal level in this situation is much higher than at the project-wide level.
Seepage is likely to be a substantial part of water requirements for the standing paddy crop. Rate of seepage is influenced not only by the character of the soil and the extent of puddling (collectively determining its permeability), but also by external factors including topography and watertable depth. On terraced slopes, seepage in upper paddies is likely to be entirely controlled by soil conditions. In the lower paddies, however, it will be influenced by seepage from up-slope areas and may be negative, presenting a drainage problem. In large areas of near-flat terrain, soil conditions will be the controlling factor early in the monsoon, but the watertable is likely to rise to the surface and limit seepage later in the season. An extreme case is provided by a near-flat area of highly permeable sand in a riverine delta. Heavy monsoon rains rapidly raise the watertable by several feet to the surface and the rate of infiltration becomes virtually nil. A late season crop of paddy is successfully cultivated.
The problem is how to estimate seepage rates for the purpose of project design. Generalized figures based on soil texture may be a useful guide for low permeability soils, but not for more pervious material, where external factors may control. The results of standard field tests (ring infiltrometer) can be entirely misleading for the latter reason. Seepage rates determined from a bunded plot several square meters in area are more relevant, although not necessarily reflecting the effects of repeated puddling, nor of seasonal rise in watertable. Better still are observations from a plot which has been under rainfed paddy cultivation for some time, in the same area, if such is available. It is noted in this connection that much nominally rainfed paddy in fact benefits from run-off (small drainage or surface flow) from adjacent uncultivated slopes. It is "semiirrigated". This fact has a bearing on the relevance of published statistical data comparing irrigated and "un-irrigated" yields (also on comparative projections of "with project" and "without project" crop production).
The above discussion refers to paddy cultivation by transplanting, which is the most common method in the South Asian area. However, paddy may also be direct seeded. This is the usual practice in Western countries, but is also being adopted in some areas of South Asia due to rising costs of labor for puddling and transplanting. Direct seeding also reduces water requirements in the initial stages of the crop, compared with the process of puddling and transplanting.
While the traditional procedure with wet-land paddy is to keep the crop continuously flooded, this is not essential. Much research has been devoted to the question of by how much crop yields are reduced if paddies are drained at intervals, and how much water can be saved by doing so. The question is particularly relevant to the rotational supply of water to paddy, which may be operationally convenient in some situations. Periodic withdrawal of water does, in fact, have some advantages. It is desirable at the time of fertilizer application (to avoid loss of nutrients with Bow from the end of the field) and it promotes oxygenation of the root zone. Published data indicates that water use can be reduced by 15 to 20% compared with continuous flooding, without significant reduction in crop yield.
However, considerably less water is regularly being used by some cultivators (e.g. Nepalese Terai), with flooding of paddies at fortnightly intervals only, on relatively high infiltration rate soils. With local varietal selection, surprisingly good yields are being obtained with this practice, which is intermediate between wet-land and upland cultivation. It must be acknowledged, however, that the conventional procedure of puddling and transplanting followed by near continuous inundation exercises very effective weed-control. Any departure from that procedure may be at the price of other means of control, although the extent of this problem varies from severe to very modest, from one area to another. The subject of "sub-optimal" irrigation of paddy warrants further investigation.