XIII. IRRIGATION AND WATER SUSTAINABILITY

-William Rite

Water as a Resource:

The importance of water for social-economic development cannot be overemphasized. It forms a vital part of social infrastructure, playing a key role in health, industry, agriculture, energy and general consumption for human welfare for which there is no substitutes.

Water is essential for all forms of life, yet the total amount of water that is available is limited. It is estimated that 80% of the countries in the world currently suffer from serious water shortages and this is expected to worsen in the coming decades.

Africa, despite its substantial water resources, experiences chronic shortages owing to uneven distribution of water and rainfall, underdevelopment of potential water resources and poor management of existing resources. In Tanzania for instance, the rise of population is creating pressure on the existing supplies and demand for additional water . The result has been overuse and pollution of the available water resources, leading to shortages even where water was previously abundant. To satisfy the projected demands, major investment in water development will be needed.

The relationship of global environment and agriculture is an important issue of concern which has drawn attention all over the world. There is a very close relationship between agriculture and the environment and in particular regarding soil and water. Ancient agriculture has been considered to be a link between nature and human society.

Therefore, sound development of agriculture based on appropriate management, control and effective use of water can play significant role in preserving the natural environment sustainably.

IRRIGATION PRACTICES

Irrigation is adding water to the soil so as to meet crop water requirements, and for the following purposes;
To cool the soil and atmosphere to make a more favorable environment for plant growth
To wash out/dilute salts in the root zone
To soften the tillage pan

Importance of Irrigation:

Agriculture is the mainstay of the majority (80 – 90) of Tanzanians. Agricultural sector contributes over 50% of the countries GDP and over 75% of export earnings (1990- 2000). But one of the most significant risk factors in agriculture is the weather. Seldom is rainfall adequate to obtain optimum crop production even in humid areas. During the growing season a rainfall does not meet crop water needs. Sound development of agriculture can be achieved through appropriate on farm irrigation water management practices.

Sources of Irrigation Water:

The main water sources are;
i.Atmospheric moisture (Precipitation)
ii.Surface water
iii.Subsurface water (underground water)

Surface sources of water:

Surface water exists in natural basins, streams and artificial reservoirs, ie. lakes, ponds, rivers and reservoirs. Water in these sources fluctuates widely from year to year and season to season.
Normally peak water demand occurs in the seasons of minimum flows. The water shortage that results has its impact during the high rainfall seasons. The parched land can’t absorb the rainfall, causing surface runoff – a waste of badly needed water.

Subsurface water:

Subsurface water is the zone of water saturation underground (aquifer), supplied by infiltration and percolation of rainfall. In areas where the underground water is a source of water supply, i.e. through wells, it is being withdrawn faster that it is replaced by the rainfall.

Challenges Facing Water Sustainability:

1.Extreme drought: depletes water from all sources (surface and subsurface)
2.Unregulated activities: development activities like farming on steep lands without soil and water conservation measures, over cutting, overgrazing and the destruction of river banks, finally lead to a serious loss of soil and water through soil erosion and drought. Soil erosion, drought and starvation are always in vicious cycle, with the water and soil loss contributing to drought and starvation which contributes to more destructive desperate activities.

Approaches to Water Sustainability:

The approaches can be grouped as follows
1.Water storage for future use:
i.Construction of big dams by intercepting water flows in big rivers (institutional and national projects) to cater for energy and irrigation purposes.
ii.Rain water harvesting techniques at farm scale through:
a.Collection of rain water in small reservoir, tanks and wells from house roofs catchments and other locally available sources.
2.Appropriate on farm irrigation water management practices, through selection, planning and design of suitable irrigation methods which are efficient, equitable and sustainable.
a.On farm rain water harvesting by soil and water conservation practices. These can be categorized as:
Biological, cultural and physical soil & water conservation measures.
i.Biological & Cultural Measures:
1.Crop rotation
2.Intercropping
3.Strip cropping
4.Trash lines
5.Tillage practices
6.Mulching
7.Organic fertilizers like farm yard manure, compost green manure etc.
8.Contour farming and ridging

ii. Physical conservation measures: i.e. terraces, cut–off drains and artificial water ways.

Terraces are cross slope barrier at suitable spacing with acceptable grades for the following purposes:
Reduce soil erosion
Provide maximum retentation of moisture for crop use
Reduce surface runoff water at a non- erosive speed
Reform land surface
Reduce sediment content in runoff-water
Reduce peak runoff rates at installations down stream
Improve formability