There are three Resources of irrigation.
Resources of irrigation
- Surface water.
- Ground water.
Rainfall is normall inadequate to sustain a very low level of agriculture production particularly in semi arid areas. Rainfall can directly help irrigation or indirectly by adding its runoff to the rivers. This runoff is then stored by weir, barrage or dam downstream. This runoff may replenish as an underground reservoir.
If direct rainfall occurs at a required time, it is very helpful for the plant and crop growth. But it is not a valid source. Because there is fluctuation in quantity of rainfall through out the year. It may vary from year to year and season to season.
In Pakistan, heavy rainfall occurs in summer season. As the temperature is high so evaporation rate is high, but still rainfall is a great boaster for agriculture. Availability of water in canal system also depends upon the rainfall and snow. In Pakistan mean annual rainfall ranges 4-30 inches in the lower indus region to the northern foot hills. Only a small portion of this rainfall makes any direct or useful combination to irrigation water supplies. According to world bank consultant report this figure ranges from 1 to 17 inches. The rest is converted to direct runoff or becomes a part of the ground water. According to estimation the present direct contribution to the crops is 6 Million acre feet per year.
Surface water includes water diverted from the streams and stored into dams and barrages. This water is then supplied to the land through canals or pumped from rivers, lakes and canals. In dry months, when snow melts, it adds a great amount of water to the river. Snow on ground provides greater storage than any man made reservoir. 1 square foot snow holds 1-4 inches of water. Snowfall occurs over many square miles on the mountaineous terrain. This snowfall then released in the summer months. The important thing for an engineer is to know when this vast quantity will be released. In Pakistan, rivers carry the melting snow and raind from the northern hills.
In rainy season, water seeps into the ground and becomes the part of ground water. This water rises the water table. Water is pumped out with the help of tube wells for irrigation. The areas where there is no access to the canal, we get water from under ground water resources. Ground water can cause water logging due to rise in water table. So water is pumped out from the ground using pumps and tube wells. In underground water, there are less chances of impurities. Underground water does not contain salts which are important for crop production. These salts act as a fertilizing agent. In Pakistan, all three resources are used. But as ground water does not contain salts, so it is mixed with surface water. So that Combined salts of both the water may not cause any damage to the crops.