Vital information on soil irrigation for rice farmers

Jan 26, 2009 Letters

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Dear Editor,

Please permit me to comment on an article entitled “Farmers requesting irrigation water despite high water level”, Guyana Chronicle, January 24th 2009.

Although average rainfall in Guyana may be adequate for the needs of the rice crop, rain comes irregularly and dry spells are frequent, such that successful cultivation of two rice crops in a year is impossible without reliable irrigation supply.

The periods of reliable rainfall are usually too short for the full growing period of the crop. Irrigation deficiency in the main season may occur either at the beginning of the crop (as in the case now) when the land must be pre-saturated, or at flowering (65-70 days after sowing) period.

For two rice crops in one year, it is necessary to adhere to a strict time schedule. This is not possible unless good control is exercised over the water factor.

In the off-season, the period with the highest water requirement (pre-saturation period) is likely to coincide with a dry or nearly dry period.

The irrigation requirements will have to be designed to meet the full water requirements of the rice field, although such design will not be required for the whole season growing period of the crop. In order to make the best use of available water resources and of financial means, it is necessary to supply irrigation on realistic water duties based on the actual water requirements in the rice fields.

Lowland rice in Guyana is grown in fields with water on the surface, and the land is prepared for sowing of the pre-germinated seeds in thoroughly wet, completely saturated conditions to obtain the muddy condition that favours good growth of the crop. The water requirements of the rice fields can be divided into four factors:

(1) Water for evapotranspiration – This includes water transpired by the plants plus water that evaporates directly into the air from the immediate environment.

The process is governed largely by climatic conditions, mainly the amount of energy available for evaporation. In flooded rice fields, evapotranspiration will never be restricted by lack of soil moisture, and will always be at the maximum possible under the climatic conditions, i.e. potential evapotranspiration.

At the latitude of Guyana, the total solar radiation at the top of the atmosphere varies only slightly over the country, but greatly over the year.

Evapotranspiration in the rice field, therefore, will be fairly constant throughout the country, but irregular throughout the year, in addition to variations due to the influence of cloudiness, humidity and wind.

Shortly after transplanting, evapotranspiration will be mainly evaporation and only slightly transpiration; but, as the crop develops, the share of transpiration will increase and evaporation decrease, until evapotranspiration is mostly transpiration when the crop is vegetatively fully developed.

The sum of both, however, is fairly constant throughout the growing period. However, not much research in Guyana has been done in this area, and thus the evapotranspiration values for irrigation season may have not been developed.

(2) Water for Percolation — As free water is kept on the rice fields for the greater part of the growing period of the crop, water loss might occur by percolation of water to the subsoil and through the subsoil to a natural or artificial drainage way.

The magnitude of this percolation loss naturally depends on the permeability of the soil layers (an impermeable layer in the profile could reduce percolation completely), the relative water levels in the field and in the drain, and the distance to the drain.

Most of the rice soils in Guyana have very high moisture-holding capacity and low volume-weight (dry bulk density). It appears that, in these soils, percolation is negligible.

(3) Water for establishing a water layer in the field – Lowland rice is almost universally grown in flooded soils. There is, however, no conclusive evidence with regard to the optimum depth of the water layer in the field.

Results from experiments in Asia showed that deep water reduced tillering and consequently decreased the number of panicles.

However, the panicle weight increased, thus the final result being hardly any difference in yield.

The practice in Guyana is that a moderate depth of 3-5 inches of water is allowed, which relatively controls weeds.

Extensive studies in this area have been carried out by the GRDB’s Rice Research Station at Burma.

(4) Water for Saturation of the soil – It is estimated that marine clays (which is prevalent in most of the front lands in Guyana) if well drained in no-irrigation periods, require an average of 10% by volume in a 4-foot profile, i.e. 4.8 inches to saturate the soil.

When the net water requirements of the rice fields have to be supplied through irrigation, allowance must be made for distribution losses that are unavoidable. So, to avoid shortages and over-irrigation, careful calculation along this line should be made.

Bissasar Chintamanie

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