Question

As part of the development of the one district, one factory programme, a catchment of size 100 km2 has been marked for vegetation. In its original condition, the average annual total runoff from the catchment is 1.1 m3/s. The average annual rainfall is 800 mm/a. In an average year, 50% of the rainfall infiltrates and 12.5% of the rainfall reaches the groundwater. Tests have turned out that the average annual evapotranspiration from the unsaturated zone (being the sum of the transpiration and the bare soil evaporation) amounts to 340 mm/a. In all water balance computations over the year, one may assume that the storage effects are small (dS/dt = +0).
a. How much water, in mm/a, reaches the root zone through capillary rise in an average year?
b. How much water, in cm/a, seeps out from the groundwater to the surface water in an average year?
c. How much water, in l/a, evaporates directly from interception in an average year?
d. How much, in m/a, is the total evapotranspiration in the catchment in an average year?
A well field is planned to withdraw 0.16 m3/s from the catchment for drinking water consumption elsewhere. As a result, the groundwater level is expected to go down and capillary rise into the root zone will no longer be possible. The percolation, however is expected to remain the same.
e. What will be the effect of the withdrawal on the different components of the hydrological cycle: the groundwater seepage, the total runoff, the evapotranspiration from the unsaturated zone and the total evapotranspiration? Please quantify in mm/a.

Answer 1

THE GROUNDWATER SEEPAGE:- The ground stores huge amounts of water and it exists to some degree no matter where on Earth you are.Lucky for people,in many places the water exists in quantities and at depths that well can be drilled into the water-bearing aquifers and withdrawn to server the many needs people have.

Large mounts of water are stored in the ground.The water is still moving,possibly very slowly,and it is still part of the water cycle.Most of the water in the ground comes from precipitation that infiltrates downward from the land surface.The upper layer of the soil is the unsaturated zone,where water is present in varying amounts that change over time,but does not saturate the soil.Below this layer is the saturated zone,where all the pores,cracks,and spaces between rock particles are saturated with water.The term groundwater is used to describe this area.Another term for groundwater is "acquifer",although this term is usually used to describe water-bearing formations capable of yielding enough water to supply peoples uses.Acquifers are a huge storehouse of Earth's water and people all over the world depend on groundwater in their daily lives.

The top of the surface where groundwater occurs is called the water table.In the diagram,you can see how the ground below the water table is saturated with water (the saturated zone).Acquifers are replenished by the seepage of precipitation that falls on the land,but there are many geologic,meteorologic,topegraphic, and human factors that determine the extent and rate to which acquifers are refilled with water.Rocks have different porosity and permeability characteristics,hich means that water does not move around the same way in all rocks.Thus,the characteristics of groundwater recharge vary all over the world.

TOTAL RUNOFF:- Runoff is that portion of precipitation that is not evaporated.When moisture falls to the earth's surface as precipitation a part of its evaporated from the water surface,soil and vegetation and through transpiration by plants, and the remainder precipitation is available as runoff which ultimately runs to be ocean through surface or sub-surface streams.Thus runoff may be classified as follows:-

1. Surface Runoff:- Water flows over the land and is first to reach the streams and rivers,which ultimately discharge the water to the sea.

2. Inter-flow or Sub-Surface Runoff:- A portion of precipitation infiltrates into surface soil and depending upon the geology of the basins,runs as sub-surface runoff and reaches the streams and rivers.

3. Ground Flows or Base Flow:- It is that portion of precipitation,which after infiltration,percolates down and joins the groundwater reservoir which is ultimately connected to the ocean.

Thus,the hydrologic cycle may be expressed by the following simplified equation:-

Precipitation (P)=Evaporation (E)+Runoff (R).

The Evapotransiration:-

1. Transpiration is the process by which water leaves the body of a living plant and reaches the atmosphere as water vapour.

2. While transpiration tkes place,the land area in which plants stand also lose moisture by the evaotranspiration of water from soil and water bodies.

3. In hydrology and irrigation practice,it is found that evapotranspiration and transpiration processes can be considereed advantageeously under one head as evappotranspiration.

4. If suffieecient moisture is always available completely to meet the neeeds of vegetation full covering the area,the resulting evapotranspiration is called potential evapotranspiration (PET).

5. Potential evapotranspiration no longeer critically depends on the soil and plant factrs but depends essentially on the climate factors.

6. The real evapotranspiration occuring in a specific situation is called actual evapotranspiration (AET).