ORA ET LABORA
HAZ LO QUE TE GUSTE
Selection of optimum dates for land and for sowing or crop establishment, and appropriate scheduling of deliveries for subsequent crop growth with a view to maximizing the use of rainfall and limited irrigation supplies is of crucial importance for the Kirindi system.
Equally important is the issue of appropriate delivery schedules at the heads of distributary and field channel turnouts that would enable a proper allocation of water to both well drained and poorly drained soils which support different kinds of crops with different water requirements; as for example non-rice crops on the well drained soils and rice on the poorly drained soils. Experience gained in System H and Dewahuwa provides us some important guidelines t o resolve this hitherto intractable problem which operating engineering staff usually shy away from.
Yala -
Dry SeasonThe optimum time for commencing land preparation for yala crops is between the fourth week of March and the first week of April when there is a high reliability of the afternoon convectional rains. For this to be possible the preceding maha crop should be harvested by the third week of March. When the onset of land preparation for maha is delayed beyond the harvest gets extended beyond mid-March of the following year and this causes severe problems for timely commencement of yala crops. The 75 percent rainfall expectancy for April is a little over 55 and this has to be captured for timely land preparation and crop establishment.
The foregoing points to the need for pursuing a policy of growing non-rice crops in maha beginning with the October rains and providing supplementary irrigation for such crops between October and Rice should therefore be restricted to the lowland soils and that too only if adequate water
is available at the source. This could be on the basis of selected tracts or distributary area within a tract.
A practice that should be fostered at Kirindi for yala as developed by at Dewahuwa is to spread the rice straw on the field soon after the
harvest in late March and burn it in-situ in the field. This enables a weed-free field which could be prepared with subsequent minimum hoeing into which grain legume seed could be dibbled in rows with the initial yala rains. For fanners who wish to till the land with a rotavator in order to prepare a better seed bed, they could avail of both rain and an irrigation issue of 70 for land moistening.
Again, based on our experience at irrigation deliveries for yala should commence in the first week of April. Two to three deliveries of 70
each at the field channel turnout within a of 14 days is adequate to prepare the seed bed and drill the grain legume crops before the New Year which falls in mid-April. In the case of land preparation and seeding for non-rice crops was in 8 days using 174 mm of delivery in yala 1985; in 15 days using 121 of delivery in yala 1986; and in 12 days using 172 of rainfall only and no delivery in yala 1987.
Daily evaporation rates increase markedly from June onwards. In order to conserve and maximize use of the limited water resource at s o m e , crop duration in the field should be as early as possible. Crop growth period commencing early April should therefore be limited as far as to around 80 days. Irrigation deliveries at seven day intervals with a 70 mm delivery ex-field channel turnout for each delivery is considered adequate for grain legumes.
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Wet SeasonPast studies on rainfall confidence limits have shown that the optimum time for sowing of upland crops in this environment is around the second to
third week of October. in
this region. However, the same land preparation methods practiced for chena crops cannot be implemented on land that is developed for puddled rice cultivation. In order to induce sufficient to the upland soil so that it could be easily tilled by conventional tillage implements, the soil has to be moistened to a moisture status close to field capacity.
To
achieve this around 40 of rain is needed to bring the upper 15 cm depth of soil to the moisture status of field capacity. The of receiving this of rain during the month of October is above 75 percent. Land preparation for seeding can therefore be completed by end of October in most years and sowing completed by the first week of November. Among the crops that are known to respond significantlyto supplementary irrigation during the season in this environment are chillie, cotton, and soyabean.This is also the traditional practice in the
In the case of rice on lowland soils, of land
will mainly depend on the inflow to the reservoir and the minimum reservoir level that has to be reached before commencement of water issues.
Scheduling Deliveries to Turnouts both Upland
Lowland
SoilBoth distributary and turnout areas consist of well drained soils on the upland and poorly drained soils in the lowland, with intervening imperfectly drained soils located in the mid-slopes. This is similar to conditions in In situations where non-rice crops are
on the upland soils and rice is grown on the lowland soils, management
are encountered in trying to achieve an equitable delivery to the contrasting cropping systems. In Mahaweli System H the problem is overcome by permitting fanners located on the upland soils and who are growing non-rice crops, to use the 1 (28.3 liters per second) discharge in the field channel during the day time for a 6 hour period each, and to permit located in the lowland soils to share the 1 cusec flow during the night under an informal arrangement
of turns.
Since there is no night storage capacity in the irrigation system, and since it will also be impracticable to open and shut turnout gates each morning and evening, the foregoing management method is the most feasible option. This method however, requires a high degree of cooperation farmers and
between agency staff who manage the turnout gates and the farmers who use the water.
It will have to be clearly understood by farmers who grow rice on the lowland that they will receive the same rotational schedule as the upland farmers and not a continuous supply for the duration of the rotation.
In the long term there should be a shift towards using the LHG lowland soils for non-rice crops during the dry yala season. The experience both in System H and Dewahuwa has been that fanners who are located at tail-end of
very long field channels on soils have been able to grow non-rice crops by providing minimal on-farm drainage. It is observed that soyabean is more tolerant to imperfect drainage conditions than most other non-rice crops, and would therefore be the logical choice for lowland soils in yala. This would, however, require a high degree of control and regulation of the field channel water deliveries in a manner that would minimize the build up of a high water table in the lowland area.