Introduction to irrigation and drainage

1.0 INTRODUCTION TO IRRIGATION AND DRAINAGE
1.1 Irrigation defined
Irrigation is any process other than natural precipitation, which supplies water artificially to the
soil to make up the deficiency of moisture under natural conditions for the profitable growth of
crops, which otherwise would not be assured.
The irrigation process involves investigation, planning, design, construction, maintenance and
operation of structures and channels for the proper conveyance of water from the source to the
point of application.
1.2 Why Irrigation?
Factors that necessitate irrigation include;
(i) Insufficient rainfall
(ii) Non-uniform distribution of rainfall.
Total rainfall during a year may be adequate to raise a crop, however it may not be evenly
distributed over the crop growth period
(iii) Control of water supply.
Irrigation enables water to be applied to the crops as and when required thereby increasing the
yield per unit area,
(iv) Growing of cash crops.
The growth of cash crops e.g. flowers, sugarcane, rice, etc. may be preferred as they bring better
returns. Such crops may require abundant supply of water and frequent watering to maturity,
which cannot be met from natural rainfall during the crop growth period. This makes irrigation
essential.
1.3 Importance of Irrigation
Includes among others the following:
• It supplements inadequate and unreliable rainfall particularly in the ASAL areas,

• It can ensure all year production hence food security.
• Helps in the control of flooding and hence erosion especially where dams exist.
• Creates employment
• Could be used to reclaim more potentially productive land and hence create more
settlement opportunities.
• Specialized production targeting high value crops is made possible
• Could be used to control pests and diseases by disrupting their life cycles.
• Could be used for strategic production as dictated by market forces.
1.4 Advantages and Disadvantages of Irrigation.
Advantages
The advantages of irrigated agriculture include;
- It increases the area of land brought under cultivation.
- It improves crop yield over rainfed agriculture three or four fold.
- Allows for the greater cropping intensity.
- It results in improved economic security for the farmer.
- Reduces risks associated with drought hence farmers can easily use high yield seed varieties,
increase the use of fertilizers, pesticides, pesticides and mechanization. They can also control
the timing of delivery of their products to the market and also the labour demands.
- It allows for the introduction of more valuable crops to farmers.
Disadvantages
The disadvantages of irrigated agriculture include
- Over irrigation leads to water logging of the soil and this reduces yields.
- Increased possibility of ground water pollution as a result of increased use of chemicals and
fertilizers.
- Due to the presence of water in open channels, there is an increased possibility of having
several water borne diseases e.g malaria, bilharzias, etc.
- It is complex and expensive undertaking; most of the time requiring donor funding yet most
of them are not self sustaining in the long run.

1.5 Drainage of Agricultural Lands
1.5.1 Defintion
Agricultural drainage is the removal and disposal of excess water from agricultural lands. It is
necessary to provide an adequate drainage system for every irrigated area.
The crop requirements are that the root-zone does not remain in the saturated state. Poorly
drained soils deprive crops of development by, among others,
- lowering soil temperature as the water evaporates.
- Stopping adequate air circulation and preventing bacterial activities.
- Encouraging certain plant diseases
- Limiting rooting penetration due to high water table
1.5.2 Sources of excess water
The sources of excess water that necessitate drainage include;
(a) precipitation
(b) Irrigation water
(c) Overland flow or underground seepage from adjacent areas
(d) Artesian flow from deep aquifers
(e) Flood water applied for such special purposes as leaching salts from the soil or for
temperature control.
1.5.3 Importance of drainage
It is necessary to provide an adequate drainage system for every irrigated area. The important
functions of the drainage system are:
1. Removal of excess ground water and salts from the soil profile in the crops root zone:
Excessive irrigation of an area over an extended period tends to raise the ground water
level. When it rises to within the capillary lift/rise of the soil, soil moisture reaches the
surface where it evaporates and in the process salts present either in the irrigation water
or dissolved in the soil, get concentrated on the land surface, leading to salinity with
harmful effects on plant growth.

Thus the two fold function of drainage is;
(i) To lower the ground water levels below the maximum depth of root zone (this helps
better aeration, plant nutrition and greater root development.
(ii) To provide an outlet for the accumulated salts which are usually washed down from the
upper soil layers and leached out thro' the drainage water. This will take place during
rainy season if there is adequate precipitation. Otherwise deliberate excess application of
irrigation water does leach the soil satisfactorily if the operation is not well managed.
2. The second function of the system is to remove excess surface water due to irrigation, likely
to occur in more than one way:
(a) In flat fields, water from either border or sprinkler irrigation collects in the surface
depressions leading to ponding. This not only lowers uniformity of irrigation but also
damages plants in the ponded areas.
(b) Excess water accumulates in the fields under border or sprinkler irrigation, at the lower
ends of the runs;
(c) Water may accumulate and flow as runoff from sloping fields under sprinkler irrigation
when the rate of application exceeds the rate of infiltration of the soil; leading to erosion
on the slopes and harmful ponding on the lowlands.
(d) Surplus irrigation water may be due to unforeseen factors as break in supply ditch or
bursting of a pressure pipe.
3. The third function of the drainage system is to remove excess surface water resulting from
precipitation. This is likely to occur in the regions with a distinct dryland wet/rainy season.
Especially, a field laid out in level irrigation borders/ basin of such a region, excess precipitation
causes logging of fields and hence the need for drainage.

1.6 Irrigation extent, potential and development in Kenya.
1.6.1 Extent of Irrigation
From a global perspective, irrigated agriculture accounts for 40% of the food production which
comes mainly from approximately 260million ha of irrigated lands two thirds of which are under
formal irrigation schemes. Out of the 11.6 billion hectares of land in 103 countries, only 8.67% is
cultivated land. The irrigated lands account for only 20% of the cultivated land world over. The
proportion of cultivated land under irrigation varies from country to country with some countries
reporting 100% of their cultivated land under irrigation, while others have practically no
irrigation. Table 1.1 shows the irrigated areas in some selected countries compared to areas
under irrigation.
Table 1.1: Proportions of arable land under irrigation in some selected developing
countries.
Country Total area
(Million
ha)
Cultivated
area
(Million ha)
Area
irrigated
(Million
ha)
Proportion
of cultivated
area to Total
area.
Proportion
of irrigated
area to total
area
Proportion
of irrigated
area to
cultivated area.
India 327.6 137.9 43.1 42.1% 13.2% 31.3%
Indonesia 149.2 14.0 7.6. 9.4% 5.09% 54.3%
Thailand 51.4 7.3 4.3 14.2% 8.4% 58.9%
Japan 37.0 6.0 2.8 16.2% 7.6% 46.7%
Philippines 30 8.3 1.6 27.7% 5.3% 19.3%
Kenya 58.2 1.7 0.11 2.9% 0.19% 6.5%

In most of the countries in developing world, the growth of agricultural production has not kept
pace with the population growth. Means must therefore be sought to increase world agricultural
production. In the arid and semi arid zones of the world, one of the principal means to achieve
this is through irrigation. The extent of arid and semi arid lands in the world is illustrated in
Table 1.2 in which agricultural production limited by drought is shown.
Table 1.2 Areas of land limited by drought.
Region % land limited by drought
North America 20
Central America 32
South America 17
Europe 8
Africa 44
South Asia 43
North East Asia 17
South East Asia 2
Australia 55
World 28
1.6.2 Overview of Kenya's agro-ecological potential

Kenya has 582,000km2 of land mass out of which 16% (93,000km2
) is of medium to high
potential. Agriculture is the mainstay of Kenya's economy. The sector contributes over 50% of
the country's export earning's and employs about 80% of the population. The sector accounts for
about 26% of the country's GDP.
Note: The countries population is currently estimated to be increasing at 2.6% annually thereby
exerting pressure on the available water and land resources causing migration of people from
high and medium potential agricultural land to low potential arid and semi-arid lands putting
pressure an already fragile ecosystem. Irrigation development is an option that will enable
intensification of land use in high and medium potential areas and integrated use of ASAL's.
This will promote future growth and development of the agricultural sector.
1.6.3 Irrigation potential and achievements in Kenya.
The irrigation potential in Kenya is 539,000 ha (based on available surface water) and a drainage
potential of 600,000 ha. Out of the total potential, 105,000 (19%) and 30,000 ha (5%) have been
developed for irrigation and drainage respectively. The potential and development by basins is
indicated below in Table 1.3.
Table 1.3 Irrigation potential and development in Kenya based on the major basins
Basin Potential ( ha ) Development (ha)
Tana 205,000 68,700
Athi 40,000 11,000
Lake basin 200,000 10,700
Kerio Valley 64,000 5,400
Ewaso Ngiro 30,000 10,000
TOTAL 539,000 105,800
Irrigation is practiced over a wide range of agro-ecological zones from supplementary irrigation
in the high and medium potential areas during the rainy seasons to near total irrigation in the
ASALs and during the dry seasons in the high and medium potential areas. Major irrigated

enterprises include; Horticulture i.e vegetables, fruits, cut flowers etc, Paddy rice, Coffee,
Food crops i.e maize, beans, sorghum etc.
The rate of irrigation development has been very low despite rising need for food and high
poverty level. Table 1.4 shows irrigated areas over the years.
Table 1.4 Progress of irrigated areas over the years.
Year 1985 1998 2005
Irrigated area 52,000 87,350 105,800
There is need for the country to increase investment to irrigation development to meet the food
requirements and improve living standards.
1.6.4 Categories of irrigation schemes in Kenya and level of development.
The categories include;
Private schemes
This category comprises schemes that are constructed, owned and managed by individual
farmers or companies. They include large estates that are run as commercial enterprises with bias
on high value crops mainly for the export market. The investment resources are privately sourced
and the schemes have their own salaried technical expertise for operation, maintenance, and
management. Main enterprises include pineapples, cut flowers, coffee etc.
Public schemes
These are irrigation schemes centrally managed by public agencies like the National Irrigation
Board and Regional Development authorities (RDA's). In the case if NIB, the investment capital
for the schemes is provided for by the government who gives the farmers tenancy rights for use
of irrigation facilities in crop production activities.
Smallholder group based schemes.

Small holder refers to schemes where the irrigation infrastructure is owned, operated maintained
and managed by the farmers either directly or through contractual arrangements with the water
undertakers.
The small holder irrigation schemes are implemented through a variety of ways that involve
partnerships between the communities, and other development agencies such as Government of
Kenya, NGO's and international agencies. Table 1.5 gives the contribution to irrigated areas by
the three categories.
Table 1.5 Comparative Irrigation Development Status
Sector Developed area ( ha )
1985 1998 2005
Smallholder schemes 17,500 34,650 47,000
National Schemes 11,500 12,000 16,000
Private schemes 23,000 40,700 42,800
TOTAL 52,000 87,350 105,800

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