Rain-fed rivers usually run dry in the months before the monsoon season. Rejuvenating a river is a complex, highly skilled and an essential process in order to sustain life and livelihoods all along its banks.
T he rivers of our country may be divided into two broad categories. Some rivers originate in the Himalayas. In addition to rainfall, these rivers also benefit from the melting of the accumulated ice cap of the mountains. The other set of rivers are entirely rain-fed, receiving water from the annual rainfall alone; in non-rainy seasons, their flow is limited to the extent of inflow of water from seepage through the soil mass of its catchment. If the rain fails, the area remains ‘thirsty’; rivers remain dry or have less water.
I will confine this article to the rejuvenation of rain-fed rivers only. There may remain possibilities of mistakes in my assumptions and analyses; this is reason enough to share it with my friends so that you contribute to rectifying mistakes and help in developing a deeper and better understanding of the rejuvenation of rivers.
M ost rivers get enough flow in the rainy season. The challenging task in the rejuvenation of a river is enhancing its flow in the dry months (from post-monsoon to the pre-monsoon months) of the year. Here, I attempt to develop an understanding of the factors that could possibly be used in planning the rejuvenation of rivers.
Let us imagine why and when rivers flow. A river cannot flow until certain conditions are met. These conditions are:
There may be other demands too; however, let us move with these minimum points.
Thus, the water lost through E and Tp is 0.45 m (0.3 + 0.15) or in terms of volume it is 22,50,000 cu m. And water that might flow as runoff to the river + the water that might get absorbed in the soil is 55% (or 0.55 m) or 27,50,000 cu m.Water holding capacity of soil:
So, the demand for filling the pore space is 20,00,000 cu m.
Thus, if the total porosity of the soil mass of the entire catchment area is allowed to be filled and no other demand is made, out of 27,50,000 cu m only 7,50,000 (27,50,000 – 20,00,000) cu m of water will go to the river as runoff (if no additional water harvesting structures are made within the catchment area).
However, about 70% (range from 60 to 80%), that is, about 14,00,000 of 20,00,000 cu m of the water gets into the soil pore-spaces in sandy, gravelly soil) and will gradually move as seepage to the drainage point as base flow.
If we assume this, 14,00,000 cu m of water will flow through the 7 dry months (post rainy months from Nov to May), that is, about 200 days, it will have, on an average, 14,00,000/200 or 7,000 cu m flow per day. This can provide about 81 litres per sec flow rate for 200 days of dry period. This will only be possible when no one is using this base flow for other purposes.
Hydraulic conductivity of the soil (sandy, gravelly texture) could vary from 5 (sandy soil) to 20 m per day. Let us assume, in this case this conductivity is 10 m per day.
This means, to have the base flow available to the stream at the end of 200 days, the farthest piece of land of 2.5 ha should be located at least 2,000 m (200 * 10) away and at a higher altitude with no barriers in the topographic sequences.
Let us look at the following sketch: Assume that the land between each successive two lines, spaced at 10 m distance, covers a narrow strip of land of about 2.5 ha. This means the entire 500 ha of catchment of the stream is divided into 200 such strips of land, located one after another in the topographic sequence.
Apart from water demand for the ecosystem, there are other demands also. These too need to be estimated to see if there is a possibility of surplus water to flow to river.Additional water demand:
The total additional demand (of people and their livelihoods + industry) is 21,00,000 (17,00,000 + 4,00,000) cu m.
If 21,00,000 cu m is taken out of 27,50,000 cu m in one go, the soil mass will receive a of maximum 6,50,000 cu m against its water-holding capacity of 20,00,000 cu m.
We know, however, that the water demand of the population is more or less uniformly distributed through the year and is generated on a day-to-day basis. To make the calculations easy, let us divide the year into three seasons, each of four months.
During the four rainy months (say July to October), only one-third of the total demand or 7,00,000 cu m of 21,00,000 cu m (Total additional water demand that is required to meet the life and livelihoods i.e. F + I) will be utilized, leaving a balance 20,50,000 cu m, to be absorbed by the soil mass of the entire catchment area. Let us assume, because it is the rainy season, that this is taken directly from the rain and not from the soil mass.
By the next four months (November to February), an additional 7,00,000 will be used by people, exhausting (un-saturating) the one-third thickness 2 m soil mass.
By the next four months (March to June), two-third of the saturated soil mass will be exhausted (get un-saturated), leaving only one-third of the soil mass saturated.
This means that by the end of the rainy season, the entire 1,00,00,000 cu m of the soil mass will yield seepage water due to hydraulic conductivity; by May, however, only one-third of the soil mass, or about 33,00,000 cu m soil mass, will contribute to seepage. Again, assuming that the soil texture is of the sandy, gravelly type, it has 20 per cent pore space, of which 60 per cent approximately will be gravitational water. This means about 3,96,000 cu m water will contribute to the base flow.
There are other complex phenomena. People start drawing water (from wells) for irrigation from about December. Till then, because of the natural hydraulic movement from October to December for about 60 to 100 days (depending on when they actually start drawing ground water), gravitational water has already travelled down the topo-sequence, that is, it has travelled about 0.6 km to 1 km downstream. Thus, by that time, water is available only in the downstream areas of the catchment within I km from the lowest part of the catchment.
In the case of more gravelly soil, hydraulic conductivity will be faster (could be 20 m a day) and the above calculations will accordingly change.
Let us build an alternative scenario (because the physical properties of the soil and other terrain features vary a lot).
|Soil Type||Unit in Litres (1000 l = 1 cu m)||Macro Porosity (in %)||Micro Porosity (in %)||Total Porosity Volume (in Litres)||Volume of Yield in Litres/Cubic Metre of Soil|
|Loam||1||These values will be somewhere between those two.|
Some research organization, somewhere in the world may have worked on and tested such an equation. I am not aware of it. My sincere request to readers of this article is to inform me about any such work done in our country. If not, we need to take up research projects on this. There has to be decentralised research across different river basins in different agro-climatic regions.
Let us assume a river has a catchment area of 1000 sq km or 100,000 ha and has been divided into 200 micro watersheds, each one of 500 ha. We need to plan for each of these 200 micro watersheds to ensure the visualized rejuvenated river. How should we approach to this? Here are some suggestive steps,
|Contribution of Each Watershed of 500 ha|
|Desired Additional Contribution to River Flow from each 500-ha area.|
|In Litres Per Second||Litres Per Day||Cubic Metre/day||% of Total River Flow Volume||Cubic Metre/Month||% of Total River Flow Volume||Cubic Metre/200 Days||% of Total River Flow Volume|
|Total desired additional river flow volume|
About the physical properties of the soil and its relationship with water holding capacity:
Now let us understand those physical properties of soil that influence the movement of water within it.
Let us also understand the range porosity of different soils responsible for water holding capacity.
Source: Plant and Soil Science eLibrary
Calculating gravitational water from the above data.
|Soil Type||Total Water at Saturation (Inch/Foot)||Available Water to Plant (Inch/Foot)||Unavailable Water to Plant (Inch/Foot)||Gravitational Water that Drains Out||Soil Particle|
|% soil volume||43.33||8.33||9.17||25.83||56.67|
|% soil water||100.00||19.23||21.15||59.62||&npsp;|
|% soil water||100.00||19.23||21.15||59.62||&npsp;|
|% of soil volume||48.33||16.6||15||16.6||51.67|
|% of soil water||100||34.48||31.034||34.48|
|% of soil volume||50.83||15.00||21.67||14.17||49.17|
|% of soil water||100||29.51||42.62||27.87|
|Value of K for||cm/sec||cm/hr||m/day|
|Fine sandy loam||0.00052||1.872||0.44928|
|Loamy fine sand||0.001||3.6||0.864|
Average permeability for different soil textures in cm/hour