Small Scale Irrigation Systems (Peace Corps) Section 6. Natural rainfall and irrigation requirements (introduction...) Rice culture and irrigation requirements

Small Scale Irrigation Systems (Peace Corps)

Section 6. Natural rainfall and irrigation requirements

Because irrigation is normally used to supplement natural rainfall, you must determine the rainfall expected by at least monthly intervals during the crop growing season. The difference between the crop moisture use by month and the natural rainfall expected lets you predict the amount of water that must be available from the irrigation system.

Normally when an irrigation system is used to supplement natural rainfall, the cost of providing the irrigation will be very high--in capital, labor to construct the system, and operation and maintenance costs. To justify the cost, it is usually necessary to increase the crop yield above that expected when moisture is the constraint.

If moisture ceases to be the constraint, then plant population (number of seeds planted per hectare) or soil fertility frequently become the next constraints. If plant population is increased and fertilizer is applied to increase crop production, then a lack of irrigation water at a critical time may lead to disaster. Hence, it is imperative that the source of irrigation water and the distribution system will supply the water required when it is needed.

In regions where irrigation is usually desirable, potentially available and cost effective, natural rainfall varies from month to month or year-to-year and place-to-place. The most easily found rainfall data will show the average annual rainfall for a location for a number of years. Data must be obtained for months and years for (or climatically near), the location where an irrigation system is being considered. Data of such detail is frequently lacking, so the National Meterological Bureau will have to be consulted and the necessary data may have to be tabulated and summarized from individual weather station reports.

Table 6-1 shows rainfall by month over 30 years for one location. Most irrigation specialists recommend designing a system to provide sufficient water for the wettest of the 20 percent driest years. There should be enough water to produce a crop in the driest year but moisture, rather than plant population or soil fertility, would be a limiting factor. With many years (30) of data available, the 20 percent driest year is found by going down the column for one month and locating the six driest years. For the month of January the wettest of the six driest years is the 20 percent driest year (1957) when precipitation was 0.13 inch; dryer years were 1953, 1961, 1964, 1969, and 1970. The same procedure was followed for the other months with the results shown for each month.

Table 6-1. Precipiation in Goodland, Kansas

To calculate the irrigation requirements, the difference between the natural rainfall for the month and the evaporation-transpiration requirements is calculated. If rainfall is less than crop requirements, the deficit will have to be made up by irrigation. Table 6-2 was prepared using Table 6-1 and Figure 5-2 for June through October, the normal growing season for sorghum in Goodland, Kansas. The second column of Table 6-2 is taken from the bottom row of Table 6-1. The third column, Water Requirements, was prepared as follows.

Table 6-2. Example of irrigation water requirements for sorghum at Goodland, Kansas. (Only the crop growth months, June through October, are considered)

Month	Rainfall, mm	Water requirements, mm	Deficiency, mm
June	34.5	60	25
July	32.5	130	98
August	16.5	210	194
September	7.9	150	142
October	3.8	38	34
Total	95.2	588	493

1/ Taken from Table 6-1.
2/ Taken from Figure 5-2, daily requirements in inches/day multiplied by days in month

Figure 5-2 shows that the water use for sorghum for June is about 2 mm per day or 60 mm for the month, for July the water requirements range from about 2.5 to 6.0 mm per day. Assuming an average of 4.2 mm per day for the month gives a requirement of 4.2 x 31 = 130 mm for the month. This procedure is continued for the remainder of the season through October 15.

Column 4 is obtained by subtracting Column 2 from Column 3, for June the deficiency is 60 to 34.5 = 25 mm. Results are rounded to the nearest mm.

Table 6-2 is somewhat conservative because it does not consider the soil moisture available on June 1. It also does not show whether during, say July, 194 mm of that irrigation water should be divided into one, two or three applications. That would depend upon soil texture and the irrigation facilities available.

The 194 mm in Table 4-2, could be applied in two applications on a silt loam soil but should be divided into three applications on a sandy-soil type.

Rice culture and irrigation requirements

Rice is the principle grain crop for most Asian countries, where it constitutes the major dietary food. In addition, rice is a commonly used grain in many African, Middle East and South American countries. Rice culture, with the exception of nonirrigated hill land rice, requires special soil, water and agronomic practices that differ from practices used on most other crops.

Numerous variables in varieties of rice, such as plant growth, grain production and tolerance to sodicsaline soils, plus variations in soil texture and structure further complicate rice culture. By relying heavily on averages, common practices and normal situations, however, the following practices generally typify rice culture.

Land Preparation. Proper irrigation of rice depends on how well the depth of water can be controlled, and how uniformly the irrigation water can be applied. A recommended practice is to use contour border levees on six (6) centimeters (cm) of elevational spacing. The 6 cm interval allows a minimum water depth of 10 cm and a maximum depth of 15 cm. Water depths of less than 10 cm are ineffective in controlling grasses, a depth over 15 cm injures rice and often submerges rice seedlings. Level and smooth the seed bed. Water leveling can move large amounts of soil, provides a smooth bed and allows for water depth measurements in the field.

The levees should have a top width of approximately 30 cm and a height of 30 cm and be well compacted to prevent breaking and seepage. Wooden gates should be constructed to control depth of water, remove stress on levees, and provide for field drainage.

Plow the field 15 cm deep and harrow to control weeds before pre-irrigating. Continual use of a uniform plowing depth on rice land builds up a "plow sole" or compacted layer. For most crops, such a layer is not desirable but for rice, it tends to seal the soil and reduce excessive water percolation (water losses) and provides some support for equipment/animals even under flooded conditions.

Pre-Irrigation-Puddling-Planting. Pre-irrigate the fields and puddle the soil with plows an arrows. Rice is usually planted as seedlings and usually by hand in most Asian countries. However it can also be broadcast or seeded by hand or with a grain drill. The soil moisture of the seed bed can be anywhere from saturated to 3 cm of water depth.

Irrigation. Start applying water three days after planting, being careful not to submerge the seedlings. If seeds are broadcast or drilled, wait until seedlings emerge and irrigate after they are 10 to 15 cm high. Increase depth of water as seedlings grow until water is over 10 cm deep. Submerge early maturin varieties continuously at 10 cm deep up to full grain development. Train field two weeks before harvest.

Some rice varieties (low land and late maturing) require drainage to the saturated soil state at the end of the first 30 days or 60 days, respectively. Then they are re-submerged until they mature.

Water Requirements. It is difficult to provide definitive figures for the tote water requirements of a rice crop from seed bed to harvest, because the amount is determined by many factors such as different soil types, growing periods, rice varieties, methods of irrigation, climate, and irrigation efficiency.

Rice usually requires 800 mm to 1,200 mm of water with extremes between 520 mm and 2,550 mm during the growing season. Daily crop water requirements are usually between 6 to 10 mm, however excessive percolation losses in sandy soils can greatly increase water consumption and also increase fertilizer requirements. But sandy soils normally are not recommended for rice culture. The table below shows typical consumption for various soil types by growing rice. The table includes evaporation, transpiration, and infiltration.

Soils	Daily Consumption (mm)
Sand	26.9
Sandy Loam	22.5
Loam	17.3
Clay Loam	14.7
Clay	13.0

In addition, the demand for irrigation increases with production (high yielding varieties require more water). Early maturing varieties (100 days) require less water per crop than late maturing traditional varieties that require up to six months for a complete cultural cycle.

It may be safe to conclude that a total of about 1,300 mm of water will be required for the complete growth cycle of rice--40 mm for seedling nursery, 260 mm for land preparation and 1,005 mm for irrigation on a clay loam soil, in a moderate climate.