Agricultural drainage is carried out by systems that intercept and convey excess water across a plot and dispose of it in a safe location. The water is transported by gravity, in a non-erosive manner, in surface or sub-surface channels. The aim is to control the specific moisture content for each type of crop and avoid losses resulting from excess water in extreme situations. The size of the system depends on the depth of the water table and the maximum volume to be disposed of, but in general channels are between 0.4 and 1.5 m deep and 0.5 and 1.20 m wide.
Agricultural drainage is implemented on plots with slopes from 1% to 25% on which there is a need to control the groundwater level or that are periodically flooded. They are particularly useful in areas subject to flooding like alluvial valleys or land with low permeability and clay or silt soils.
Threats and Impacts it Addresses
Drainage avoids crop damage from intense rain and flooding by removing excess water. It also controls soil moisture content, allowing crops to grow properly, which increases productivity and, consequently, food security. The moisture retained during the rainy season may be beneficial during the dry season.
- Identify the areas on the parcel where water naturally runs off.
- Identify the type of drainage to be used (surface or sub-surface).
- Calculate the depth, width and length of the drainage system according to the source and amount of water to be removed, the problem that it creates, the permeability of the soil and the type of crop that the measure is intended to benefit.
- Excavate the ditches with an inclination that will avoid the accumulation of sediment and allow the runoff to flow at a speed of at least 0.25 m/s.
- Complement surface drainage with compacted ridges and sub-surface drainage with filler consisting of rocks or gravel in addition to installing runoff pipes.
Inputs and Costs
The cost of constructing a surface drainage system 0.5 m wide, 0.8 m deep and 1000 m long on approximately 5 ha of land is given below. The main inputs are the labour to excavate and move material as well as to analyse soil, precipitation and runoff conditions. Two days for training in constructing and operating the system are considered.
| Surface drainage system 0.5 m x 0.8 m x 1000 m
Economic and Ecosystemic Benefits
Drainage systems save energy used in irrigation by controlling soil moisture. They prevent crop loss from flooding and maintain agricultural soil conditions so as to maximize yields. They eliminate excess water on farmland and control the groundwater level to ensure the best balance of water and salts in the crop root zone (Pizarro, 1985). Polón and others (2011) report that in heavy soils that have been improved by drainage, production increases ranged from 50% to 100% for cereals and from 90% to 200% for tubers. The benefits of the measure are seen at the end of one annual cycle.
Agricultural drainage cannot be implemented on flat land where runoff cannot be safely disposed of. Underground drainage is more costly by orders of magnitude than surface drainage. Drainage channels transect plots, which makes it more difficult to use machinery and limits access in general.
It is important to start with the smallest number of drainage channels possible and to complement the hydraulic infrastructure with simpler measures, such as contour trenches and keyline ploughing, to promote infiltration of excess runoff. The dimensions of the system are important because improperly designed drainage can parch the land.
Drainage systems require maintenance and constant monitoring. They may be complemented with absorption wells, water storage tanks and pumping systems. Many of the problems of agricultural soil could be mitigated with appropriate drainage systems, but the proper operation of the system and its limitations must be understood.
Units to Monitor Project Progress: Length of drainage systems constructed (m).
Unites to Monitor Measure's Impact: Increase in productivity (t/ha); area protected with agricultural drainage (ha).
- Pizarro, F. (1985). Drenaje Agrícola y Recuperación de Suelos Salinos. Madrid: 2nd ed, Editorial Agrícola Española, S.A.
- Polón Pérez, R. and others (2011). "Principales beneficios que se alcanzan con la práctica adecuada del drenaje agrícola", Cultivos Tropicales, vol. 32, No. 2, pp. 52-60. Available at: http://www.redalyc.org/pdf/1932/193222422010.pdf.
- Ayers, R. and D. Westcot (1985). Water Quality for Agriculture. Rome: FAO Irrigation and Drainage Papers No. 29.