To design a drainage system, which should be known is the amount of water that must be removed from the land in a certain period of time, this is done to avoid a rise in surface water. Discard the excess water in the soil has two benefits: (1) prevent puddles on land (2) remove water from the root zone, so that the salts brought by irrigation can not reach concentrations that would be harmful to plants.
To determine the magnitude of the rate of drainage of several factors to consider, among others, (1) The diverse natural conditions, and (2) the amount of water to be discharged. Therefore, the field work to be done to figure out how soil conditions, water conditions, salinity and the amount of water that must be maintained. To calculate the need for drainage, water balance analysis should be done on the whole area will be in the drainage as shown in Fig.
Figure 30. The concept of the balance of water in soil
Water balance is usually counted in the average term of one year. Waterlogging and salinity problems should also be considered in-take into account the water balance. Especially for certain years, for example, a very dry year or a year with extreme rainfall, or even for a certain period, for example the growing season or rainy season.
Surface Groundwater
Ground water table is the upper limit of the groundwater. It is defined as the point where the water pressure in the ground is equal to atmospheric pressure. Beneath the surface of groundwater, all soil pores filled with water, al is known as saturated zone as shown in Figure 8:13.
Figure 31. Surface soil water
Generally, groundwater flow toward the drain occurs in the saturated zone. On the surface of the water, there are zones where soil pores filled partially with water and most of the air, and the zone is the unsaturated zone. Water in the unsaturated zone from rain or irrigation water that has entered into the ground, and that comes from the capillary rise of groundwater. Unsaturated zone is essential for plant growth. In this zone the roots of the plants take water from the soil.
High water level is always changing with time, after irrigation or rainfall occurs will increase the water surface and is followed by a gradual decline due to the flow of water into the drainage system.
Water levels be measured to determine the highest water levels (maximum), average and minimum or lowest annually. Ground water levels are constantly changing due to the replenishment of groundwater by rain or irrigation.
For this reason, the measurement of ground water level should be performed at frequent intervals, at least for a period of one year. The interval between readings should not exceed one month, but two weeks is probably better. All measurements of ground water level in the drainage area that will be done as early as possible, and conducted over a span as short as possible so that a complete picture of the surface of the water in that time frame can be obtained.