1. Increasing effective rainfall
2. Increasing the effectiveness of rainfall
3. Further lines of work needed in the field of effective rainfall in agriculture
Rainfall is a free source of water and its proper utilization will lead to increased crop production. Farmers and researchers, planners and administrators, have been engaged from early times in the task of increasing the effective proportion of the total rainfall and also its effectiveness for increased crop production. he main practices in this respect can be listed as follows:
1.1 Reducing Surface Run-off
1.2 Increasing Infiltration (except in rice culture)
1.3 Building Water Storage Structures
1.4 Minimizing Peep Percolation Losses
The approach to increasing effective rainfall in agriculture includes:
This is achieved by altering the topography of land, erecting barriers to the flow of water and by increasing the opportunity for infiltration. The practices followed include:
- land grading and levelling;
- bunding, terracing, basin listing, criss-cross ridging;
- retaining crop residues after harvest;
- reducing depth as well as frequency of irrigation.
Increased infiltration is achieved by improving the soil structure or condition at the surface, as well as in the subsurface. The practices include:
- deep ploughing, sub-soiling or breaking hard pans;- adding organic matter and soil conditioners to improve the texture of the growing pasture or grass legume mixtures;
- using mulches to prevent breaking of aggregates which seal the soil surface;
Farm ponds at suitable places are very useful to collect run-off water for use during periods of low rainfall at critical stages of crop growth.
Deep percolation losses can be minimized by practices such as;
- increasing the water holding capacity of light textured soils by the addition of clays or organic matter, if this is economically feasible;- indirectly by extending the root zone though selecting deep roted crops;
- reducing permeability beyond the root zone, especially in rice, using soil conditioners (krylium) and placing plastic sheets and bituminous layers at some depth in the soil.
The losses due to deep percolation can be very high in rice fields and may amount to 50 to 75 percent of the water applied. It is here that a great impact can be made in economising on irrigation water or in increasing the efficiency of rainwater by adopting the various measured listed (Dastane et al (1970)).
- select for the rice growing heavy soils or those with hard pans and shallow depths;- grow crops in large blocks instead of isolated fields, since the seepage can be inversely proportional to the perimeter of the area;
- scrupulously level the land to avoid excess water applications;
- puddle and destroy structures in the surface soil;
- compact soil, embed polythene sheets and apply chemicals such as bitumen and asphalt, or a cement-concrete layer;
- construct sub-soil dams to arrest horizontal flows of water;
- apply light instead of heavy irrigation in irrigated areas to accommodate rainfall; carefully follow a water budgeting procedure.
The object of measuring the effectiveness of rainfall is -so increase production per unit of water, per unit of land and per unit of time. This can be achieved by planning suitable crops and carrying out farm operations consistent with the amount, intensity and frequency of rainfall. These include;
- planning cropping patterns consistent with rainfall patterns;- selecting high value crops for maximizing the income;
- adjusting sowing times according to the probable rainfall so that drought sensitive stages of growth do not synchronize with periods of inadequate rainfall;
- timing other farm operations such as tillage, fertilizer application, applying insecticides, fungicides and herbicides etcetera, in relation to rainfall probability periods;
- following mixed cropping of shallow and deep rooted crops for full exploitation of stored soil moisture;
- adopting 'multi-storied' cropping practices, water needed for a single crop and for several crops together is more or less the same, but total production can be much. higher in the latter system than in the former;
- improving weather forecasting techniques and using the predictions in planning farm operations.
3.1 Collecting Data by Setting Up Crop Lysimeters
3.2 Verification of Empirical Methods
3.3 Development of Empirical Methods in Different Areas
3.4 Utilizing Information of Effective Rainfall in Agricultural Practices
3.5 Increasing Effective Rainfall Under Field Conditions
3.6 Increasing the Effectiveness of Rainfall
Very little experimental information is available for planning purposes on the region-by-region values of effective rainfall in agriculture. This data should, therefore, he collected. The setting up of crop lysimeters at suitable places would go a long way towards achieving this. Agricultural research stations, agro-meteorological stations and irrigation scheme headquarters may be selected for such studies. The staff can work out the corrections which should be applied to the value of total rainfall for the different conditions, such as for leaching requirements, for shallow water tables, and different crops. Effective rainfall is an entity governed by site and situation, hence systematic collection of data and judicious interpretation should be done on a small region-wide basis. Additional field studies should be made to verify the collected information under field conditions. In this respect, several techniques are available to study water balance parameters. One of them, lysimetry is rather costly and requires competent personnel. However, it is the most reliable technique. Alternatively, water balance studies can also be conducted in small watersheds in representative irrigated areas.
Empirical methods for estimating effective rainfall are simple and practical, and have a high extension value. However, these methods should be verified in individual areas against the standard methods and improved, if necessary, by applying suitable corrections*
Rapid methods must be developed for estimating effective rainfall in all irrigated areas. Empirical methods with suitable assumptions would be the best initial approach because of their inexpensiveness and reliability when corrected for local conditions.
Studies on effective rainfall in rainfed farming areas will reveal to what extent water is used at the site and to what extent it is wasted. These will facilitate the drawing up of plans for more efficient utilization of water resources.
Information can be used productively and profitably in all irrigation projects. In new schemes, water deliveries, both in time and depth, should be planned on the basis of short period values of effective rainfall. Often either no values or generalized values are taken for the entire season. Planning, management and operation of irrigation schemes on the basis of carefully computed effective rainfall data would increase agricultural productivity and economize on irrigation water usage.
Effective rainfall can be increased by reducing surface run-off, increasing infiltration and reducing deep percolation losses. Several practices are listed, but they are often not followed because their usefulness is under-estimated or their cost is too high.
It is therefore necessary to popularize them through extension agencies. What can be done at the individual, community and district levels needs to be defined and demonstrated.
Rice culture offers an enormous scope for increasing effective rainfall by reducing percolation losses ranging between 50 and 75 percent. Studies using techniques of puddling, compacting, adding chemicals, lining the sub-soil with impervious materials and constructing sub-soil dams, would go a long way towards reducing the crops' water needs.
For lack of studies on the subject, very little progress has been made in developing practices for increasing the effectiveness of rainfall. Economic studies on the value of rainfall in terms of the production and benefits should be made for its proper appraisal and further augmentation.
