13 River Water Yield Estimation

Ranjana Ray Chaudhuri

epgp books

 

 

 

Objectives:

  • To assess the surface water volume using drainage characteristics of the catchment and river channel morphometry
  • To assess the yield over a chosen period

 

Introduction

 

The river water yield is the total quantity (volume) of surface water that can be expected at the outlet of the river or stream where the stream leaves the catchment in each period. In case of a river the annual and seasonal flow are considered important, the yield is measured as annual yield, if the period chosen is one year of flow through the catchment and if seasonal yield is to be estimated then the specific seasonal flow quantity is estimated, that is flow in summer months or monsoon months or winter months. Annual yields over many years constitutes the time series of annual yield. From this the average annual yield, 75% dependable yield, 50% dependable yields are calculated. This means that 75% or 50% of the times this yield is expected to be carried by the river (so 15 out of 20 years and 10 out of 20 years these yields are expected in the river respectively). The yield corresponds to natural flow in the catchment, but nowadays return flow from agricultural fields and industry are also expected to be taken into consideration at the outlet of the catchment where the estimation is made.

 

River water yield is measured as stream flow or discharge in cubic metre per second (m3/s, volume over a period of time), along a defined stream profile. This yield is contribution of water from all sources upstream of that point in the river and the catchment contribution. The river is also the unit of the hydrological cycle which carries water from the mountains to the ocean through the land surface. Thus, river water yield estimation includes a combination of overland flow (which is the flow or runoff from over land after infiltration, interception and depression storage has been accounted for in the catchment after a rainfall episode), interflow which is sub surface through soils below the surface through soil pore. The flow is because of gradient of the land, the third component is base flow, which is the contribution of ground water flow. The subsurface flow and overland flow together comprise quick flow, as this is the runoff during and immediately after the rainfall event has occurred. This is also called the new water in the catchment. The contribution from groundwater to a river is otherwise called old water as it reaches the river much later after the rainfall episode is over.

 

The river water yield at a point in the stream is a function of both time and space. This means that the stream flow varies in the stream on hourly basis, daily basis, monthly basis, yearly basis and where the point is in the catchment and where is the catchment located in the river basin. The river water yield is a process as it receives water after a rainfall episode in the catchment and a part of the storage component in the hydrological cycle (as lakes etc.). The yield depends on the sequence of rainfall events, has it been raining for three days/five days in the catchment, what was the intensity of rainfall, the seasonal distribution, the areal extent of the rainfall. It also depends on evaporation and transpiration from the catchment, that is the vegetation cover plays a part, the soil type which governs infiltration capacity, the rock type which governs deep percolation and groundwater contribution. Due to human activities river water yields have got modified due to withdrawals for irrigation, for industry, inter basin water transfer, return of flow to stream as waste water.

 

The river channel form or profile also plays a major role, it is shaped by catchment characteristics, the stage of the stream. The river water yield estimation is done with the help of mathematical and statistical models (explained further in methodology).

 

The river water yield is important as rivers are the most important source of surface water. The yield depends on the basin characteristics and catchment characteristics. The morphometric conditions of the stream influence the stream flow. The meaning of morphometry is in two parts, morpho means form and metry means measurement. This includes the gradient, slope of the basin and relief amongst other characteristics. The relief of a river is governed by tectonic activities, formation of fold mountains and geological characteristics of the area. The geomorphology of the area influences the hydrology of the basin and therefore influence on stream flow. Currently, anthropogenic activities in the river basin also influence the flow, for example the presence of dams, smaller check dams and other interventions at catchment level like rain water conservation strategies. These affect the downstream yield in the river. The amount of water flowing down a river also depends on the drainage density of the catchment, which is defined as the total stream length divided by the catchment area. The stream frequency is another important factor, the number of streams present in the catchment divided by the area of the catchment represents stream frequency. It depends on the nature of landscape, nature and type of rainfall, type of rocks, soil permeability. For example, if the soil is permeable, infiltration is high and stream frequency is not high. These factors determine the discharge in the river and another important factor is whether the river originates in the catchment of carries flow of snow melt and base flow from a previous catchment.

 

The stream order of a stream depends on what kind of a tributary it is. If the stream has no tributaries, it is a first order stream, two first order streams meet to form a second order stream, and this is how higher order streams are established. So, a stream of order 9 or 10 is expected to be a large stream. The variation in the stream order and catchment size depends on physiography and structural conditions. For example, if a river basin develops on soft rocks like shale or slate then the chances of having large number of streams due to erosion are high.

 

Length of runoff or overland flow before it meets the stream, it is measured as number of kilometres per square kilometer. If the value is lo, for example 0.5 kilometre per square kilometer, this means the slope is steep in the catchment and rain water will enter the stream quickly.

 

The drainage density is defined as the ratio of the total length streams of all orders taken together divided by the catchment area. Drainage density varies from 0.55 to 2.10 km/sq.km, low drainage density reveals that the soil sub surface may be permeable with good drainage properties, while high value of greater than 1.5 reveals that the overland flow will be high. The drainage pattern can be tree like (dendritic), herringbone type or tributaries may be parallel to the main stream in a basin, this determines the total flow through the main stream

 

It is important to understand catchment characteristics as each catchment has unique features, for example a river valley between mountains as a very steep gradient with thick forest cover, the rocks at the base of the river may be of tectonic origin, if the average bed slope is 50m/km it signifies that it will be fast flowing river which may not have too much yield yet, the velocity will be high. In the hills many times the tributaries meet at right angles to the main river, if the structure of catchment is longitudinal which is the case in mountains, while in the flatter reaches of the river the basin is more rounded or oval. The peak flow in the river after snow melt or heavy rainfall is governed by the topography. These concepts have been developed by Horton and later modified by Strahler, as is discussed in books on hydrology (Maidment, D. R. Handbook of Hydrology, Mc Graw Hill, New York, 1992; Subramanya K. (2008), Engineering Hydrology, third edition, Tata McGraw Hill Publishing Co. , New Delhi, India).

 

The cross section also known as cross profile of the river varies from narrow, steep slopes (down cutting), deep channels in hills, near the water source to relatively gentle slopes in middle course (lateral erosion) of the river, with lesser depth, while as the river reaches its lower course, the slopes become gentle, wide channels with shallow depths (alluvium deposition), as explained in figure 1 and figure 2 below.

The relationship between geomorphology and hydrology may be explained with the help of the following models. These relationships use hydrologic and geomorphological variables to understand the influence of the channel geometry and landscape on the channel flow. These formulae as mentioned below for river water yield estimation at regional level. The first two formulae are related to the channel cross section; the shape and size of the stream/channel govern the yield estimation.

Q2.33=3.741.Xs.Rasp-0.515 (1)

 

Where Q2.33= the mean annual flood flow in m3/s, this mean that it has an average recurrence interval of 2.33 years,

 

Xs=channel cross sectional area measured in m2

 

Rasp=ratio of maximum depth of channel to mean depth of channel

Q=0.129.Xs1.157.Rasp-0.781 (2)

Where Q= the mean annual discharge in m3/s, it is the average flow in the stream taken over a time period of 30 years

Xs=channel cross sectional area measured in m2

Rasp=ratio of maximum depth of channel to mean depth of channel

Q2.33=12 A0.79(3)

Where A corresponds to the drainage area of the catchment in km2

Q2.33=1.48.10-4.A0.82.P22.18(4)

Where A corresponds to the drainage area

P2 corresponds to the daily rainfall (24 hour) with 2 year return period, measured in mm

 

Formula 3 and 4 determine discharge in terms of drainage area and duration of rainfall, so they take the catchment characteristics into account. Many other formulae also exist which are made catchment specific or channel geometry specific.

 

[All the four formulae (1-4) are taken from Handbook of Hydrology by David R. Maidment,Mc Graw Hill Publication,New York,1992].

 

The channel reach in which the discharge is determined to find out the yield, it is important understand the geometry properly as the gradient of the channel bed and plan affect the flow velocity, travel time and sediment transport capacity. Though sediment transport capacity has not been discussed in detail in this module, yet in case of flood flow it assumes it becomes significant. They depict that velocity of flow in the channel, the width and depth (as in the shape of the channel) and channel bed slope affect the mean annual discharge in the channel. Similarly relationships may be established between hydrologic variable mean annual flood flow in m3/s and drainage density, ratio of relief to length and drainage area and sediment yield. Such relationships may be established for all streams and river yield may be determined.

 

Another commonly used model in India is the Khosla’s formula (taken from Engineering Hydrology, K.Subramanya, Fourth edition, 2008).

 

In this the catchment climate conditions are taken into consideration (mainly temperature), often as due to lack of data no other parameter value is available readily.

     Rm=Pm-Lm (5)

And Lm=0.48Tm for Tm>4.5 degree centigrade (6)

 

Where Rm=monthly runoff in cm and Rm>0

 

Pm=monthly rainfall in cm

 

Lm=monthly losses in cm

 

Tm=mean monthly temperature of the catchment in 0C

 

For Tm<4.50C, the loss is assumed minimal

 

The annual runoff is a summation of all monthly runoffs, it has been tried in many catchments in India and found to give good results (the losses Lm component takes into account the infiltration, evapotranspiration etc. in the catchment).

A solved example on Khosla’s formula:

 

In a catchment in India, the average monthly rainfalls and temperature are given, estimate the annual runoff from the catchment using Khosla’s formula.

 

Conclusion

 

Due to anthropogenic influences and climatic changes like rise in temperature, glacial melts will increase the flow in rivers initially and then due to shrinking glacier size, the quantity of water in the rivers will reduce. Sometimes this may be abrupt change, so monitoring and estimating the river yield is more important now. It needs to be done on regular basis as river yield will influence future planning. Deforestation in the upper reaches increases the erosion, the sediment levels in the river increases, the water carrying capacity of the river reduces, leading to lower yield values in certain sections of the river.

 

The location of dams in upper reaches of the river is influencing the river yield in the lower reaches. This becomes critical if the rainfall is below normal in the lower reaches and thus the contribution to river flow due to rainfall in the catchment is reduced.

 

Estimation of yield is very important for allocation of water to the various sectors like irrigation, industry and urban areas. This is because river is the basic unit which allows water to be stored in reservoirs and supplied through canals for anthropogenic activities. In times when the flow is reduced in the stream then the minimum annual seven day mean flow in the river becomes very important to know. So if the annual assessment is available such data can also be extracted and adaptive measures can be suggested as reduced yield values will also be available.

 

Actual measurements in the stream are carried out at stream gauging stations with the help of staff gauges, weirs, as explained in the modules on water measurement techniques) determine the flow. In case of the catchment characteristics like drainage density, area both techniques of Geoinformatics Systems (GIS) and land surveys are carried out. Together with this it is important to monitor the ground water yield in a catchment as it has been seen that excessive extraction of ground water from the catchment reduces the base flow in streams/rivers so the seasonal yield in the summer months goes down in rivers. This in turn makes rivers dry and then the catchments down stream of such locations face water crisis. For maintaining sustainable river water yields it is important that sustainable ground water yield is followed. Else in the long term both ground water and river water quantities will be reduced, especially in deficit rainfall years. Already certain parts of the country which are in semi-arid and arid ecosystems, water scarcity is noticed. Ground water stays in the hydrological cycle for a much longer period of time because it is a very slow flow, however, once depleted it is very difficult to replenish. That is why rivers which are dependent on base flow completely in summer months, for example rivers of South India, due to excessive use of ground water in the upper catchments, no base flow reaches the lower catchments and the river runs dry. Under such circumstances, many times the river is left with only treated waste water flow from cities which eventually reaches the river through various drains. If we clean this water to required standards, then the river will have reasonable good quality water and yield even during summer months and the eco system of the rivers shall survive.

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References

  • Blench, T. (1957). Regime behavior of canals and rivers. Butterworth’s, London.
  • Leopold, L.B. and Miller, J. P. (1956). Ephemeral streams- hydraulic factors and their relation to drainage net. U.S. Geology Survey Water Supply Paper 282A.
  • Maidment, D. R. (1992). Handbook of Hydrology, Mc Graw Hill, New York.
  • Mosley, M. P. (1979). Prediction of hydrologic variables from channel morphology, South Island Rivers. Journal of Hydrology (New Zealand) Vol. 18, No. 2 pp. 109-120.
  • Mosley, M. P. (1981). Semi-determinate hydraulic geometry of river channels, South Island, New Zealand. Earth surface processes and Landforms, Vol. 6, pp. 127-137.
  • Natural Environment Research Council (NERC). (1975). Flood Studies Report. Vol 1. Hydrological Studies NERC.
  • Ojha, C.S.P., Berndtsson,R. and Bhunya, P. (2008). Engineering Hydrology, Oxford University Press.
  • Rodda J.C. (1969). The signifcance of characteristics of basin rainfall and morphology in a study of floods in United Kingdom. UNESCO Symposium on floods and their compilation, Vol. 2, pp. 834-845.
  • Schumm, S.A. (1986). River adjustment to altered hydrologic regimen Murrumbidgee River and Paeleochannels Australia. U.S. Geology Survey Prof. Paper 598.
  • Subramanya K. (1997). Flow in open channels, second edition, Tata McGraw Hill Publishing Co, New Delhi, India.
  • Subramanya K. (2008). Engineering Hydrology, third edition, Tata McGraw Hill Publishing Co., New Delhi, India
  • Thomas, D. M and Benson, M.A. (1970). Generalization of streamflow characteristics from drainage basin characteristics. U.S. Geology Survey Water Supply Paper 1975.

 

Sources of figures:

 

Figure 1

 

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Figure 2

 

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