Friday, April 11, 2008

Rain Water Harvesting

NOTES ON IRRIGATION ENGINEERING 1
CODE: 161

TIME (HOURS): THEORY 24
PRACTICAL 36
TOTAL: 60

AIM: To teach students water sources, water harvesting and water abstraction techniques so that they can be able to apply the knowledge and skills to practice and advise farmer on better methods of water storage and efficient utilization.

OBJECTIVES: Students should be able to: -
1.Design an Irrigation scheme
2.Sdvise farmers on proper utilization of water resources for improved land productivity

Major topics
1.WATER RESOUCES
2.RAIN WATER HARVSTING
3.WATER STORAGE (RESERVOIRS)
4.DAMS
5.WATER ABSTRACTION
6.CONSTRUCTION MATERIALS
7.MORTARS
8.CONCRETE
9.REINFORCEMENTS


INTRODUCTION
Irrigation is the artificial application of water to the soil for the purpose of crop production. In regions in the world where precipitation during the entire or part of the growing season is insufficient for the growth of plants, irrigation is as old as agriculture.

Although the start of irrigation in the various subtropical regions cannot be determined, indications exist e.g. in Egypt about 4000 B.C, crops were irrigated. The first development in China occurred around 500 B.C.

An increase in the irrigated surface in 1850-1940 was evidently related with the increase of population and the expanding international trade; so that both more food and industrial crops were needed.

Development in Tanzania
The water development and Irrigation Division (W.D & I.D) was formed in 1946.Charged with the responsibility of conserving the natural water resources of the country and the provision of water for domestic consumption, Livestock and development of small-scale irrigation schemes in rural areas. Traditional irrigation has been confined to mountain areas where permanent streams are available such as Kilimanjaro, Pare, and Usambara mountains.
By 1973 the area under irrigation in the country was about 4000 ha. Limitations were bilharzias, water logging, malaria and salinity.

Irrigation as a subject can be divided into Irrigation agronomy, which takes care of the crops and soil, water management and Irrigation Engineering that take care of the design of works in connection with river controls, irrigation structures, drainage of water logged areas, storage, division, lift of water. Irrigation Engineering one is just part of it


WATER SOURCES
A water source: Is a place where water can be found for different uses. Basically rain and snow are the sources of all water, and the source of almost of our rainfall is the sea. Water falls on the earth’s surface in the form of snow, hail, rain or drizzle and condenses upon it as dew. This water is known as precipitation originates from water vapour in the atmosphere.
The major source of irrigation water can be grouped into two: -
(i). Surface water source: - These are exposed to the atmosphere and they refer to e.g.rivers, lakes, streams, ponds, ocean, reservoir etc
Surface water in general implies the utilization of river water and if the diversion from the river and the transportation to the fields is taking place by gravity, such a system is indicated as gravity irrigation.
















Fig. Water surface elevated by a guiding dam

East Africa is fortunate for it contains a large proportional of total area of fresh water on the African continent. It includes the great lakes but also numerous smaller lakes, swamps etc, as well as several thousands kilometers of permanent and seasonal rivers.

Table: Approximate area of lake in Tanzania. Source; B.Lundgren (Land use in Kenya and Tanzania) 1975.

Lake system
Area (Km2)
Max.depth (m)
Bahi (seasonal)
155
2
Eyasi
1035
2
Jipe
20
-
Kitangili
155
5
Malagalasi (seasonal)
9325
2
Manyara
450
2
Natron,Ngareng;ro (seasonal)

2
Nyasa
7500
1400
Rukwa
2850
10
Tanganyika
13200
1400
Victoria
34500
75
TOTAL
69190



With this total Tanzania accounts for about 30% of the total lake resources of the African continent. Some of the lakes listed however are alkaline e.g. Natron.Thus limited utilization value.

(ii). Ground water sources: -
Under this are the wells, springs, and bore holes can be well studied under ground water hydrology, which is the science of occurrence, distribution and movement of water below the surface of the earth. The largest available source of fresh water lies underground. The total groundwater potential is estimated to be one-third the capacity of the ocean.
The main source of ground water is precipitation. A portion of rain falling on the earth’s surface into ground travels down and when checked by impervious layers to travel further down, forms ground water. The ground water reservoir consists of water held in voids within a geological stratum. The sources of ground water include water from deep in the earth, which is carried upward in intrusive rocks, and water, which is trapped in sedimentary rocks during their formation. The quantities of such water are small and they are often so highly mineralized as to be unsuitable for use. The discharge from groundwater occurs in two ways:-
(i). Natural way; which occur as flow in lakes, reservoirs, rivers, ocean and springs.
(ii). Artificial way; which is by pumping from
wells