The water balance of the Betwa basin, India / Le bilan hydrologique du bassin versant de Betwa en Inde

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1 Hydrological Sciences Bulletin ISSN: - (Print) (Online) ournal homepage: The water balance of the Betwa basin, India / Le bilan hydrologique du bassin versant de Betwa en Inde. V. SUTCLIFFE, R. P. AGRAWAL & ULIA M. TUCKER To cite this article:. V. SUTCLIFFE, R. P. AGRAWAL & ULIA M. TUCKER () The water balance of the Betwa basin, India / Le bilan hydrologique du bassin versant de Betwa en Inde, Hydrological Sciences Bulletin, :, -, DOI:./ To link to this article: Published online: Dec. Submit your article to this journal Article views: View related articles Citing articles: View citing articles Full Terms & Conditions of access and use can be found at Download by: [...] Date: anuary, At: :

2 Hydrological Sciences-Bulletin-des Sciences Hydrologiques,,, / The water balance of the Betwa basin, India Downloaded by [...] at : anuary INTRODUCTION. V. SUTCLIFFE Institute of Hydrology, Wallingford, Oxfordshire X BB, UK R, P. AGRAWAL Central Ground Water Board, New Delhi, India ULIA M. TUCKER Institute of Hydrology, Wallingford, Oxfordshire X BB, UK ABSTRACT A study of the water balance of a basin in India, where the annual monsoon season of water surplus contrasts with a longer period of deficit, shows that estimates of soil moisture recharge and groundwater recharge may be obtained in these circumstances by comparing seasonal net rainfall with runoff on two assumptions: soil moisture recharge is constant from year to year, and groundwater recharge is proportional to runoff. Le bilan hydrologique du bassin versant de Betwa en Inde RESUME Une étude du bilan hydrologique d'un bassin versant en Inde, où la saison annuelle de la mousson avec un excès d'eau contraste avec la période suivante déficitaire, montre qu'on peut estimer les apports absorbés par le pouvoir de rétention du sol et la recharge des nappes dans ces circonstances par la comparaison de la hauteur de précipitation nette saisonnière avec l'écoulement en admettant deux hypothèses: l'alimentation de l'humidité du sol est constante d'année en année et l'alimentation des eaux souterraines est proportionnelle à l'écoulement. The Betwa Project was a research programme executed by the Central Ground Water Board in cooperation with the UK Natural Environment Research Council. A main aim was to assess the groundwater recharge and thus the groundwater potential of the basin of the upper Betwa River, at the same time developing techniques which could be applied to similar areas elsewhere. The aims of this paper are two-fold: to provide an account of the water balance as a framework for detailed studies of the hydrological processes and of the hydrogeology; and to attempt to estimate the seasonal soil moisture storage and groundwater recharge by water balance methods using data existing at the start of the project. The first aim may be considered as providing a vital but supporting part of the whole study. The second aim is more ambitious; although the approach was thought unlikely to give precise results as it depended on analysing the differences between estimated quantities, the

3 .V.Sutcliffeefa/. highly seasonal climate has made it possible to develop a simple method of estimating soil moisture recharge, and less reliably, groundwater recharge. This may be usefully applied in similar climates elsewhere to obtain preliminary estimates before detailed investigations begin. THE ENVIRONMENT Downloaded by [...] at : anuary The monsoon climate of the Betwa basin provides a large seasonal water surplus of rainfall over evaporation, which either runs off or recharges the subsurface reservoir. By measuring or estimating the rainfall, the evaporation and the runoff, it is possible to make an estimate of the total recharge. However, it is necessary to distinguish between the soil moisture reservoir from the surface to the bottom of the root zone, and the groundwater reservoir which drains below this; though the two are continuous and the boundary may be altered by changes in land use, the first is available for evaporation during the dry season while the second drains beyond the reach of roots and eventually contributes either to deep storage or to maintain the baseflow or dry season component of river discharge. Thus under natural conditions the soil moisture replenishment eventually evaporates, while the groundwater recharge does not; in agricultural terms the soil moisture replenishment provides water to the crop roots during the natural growing season, while the groundwater recharge could be exploited by irrigation to extend the growing season. The Betwa, a tributary of the amna, drains from the Deccan plateau. The catchment above Dhukwan Dam, where the longest flow records exist, is km. The basin is saucer-shaped, with sandstone hills around the perimeter and clays underlain by Deccan trap basalts in the lower ground towards the centre; granites are exposed near the northern end of the basin. The total range in elevation is only from about to m a.s.l., but the topography and vegetation vary in a fairly complex pattern from flat open wheat-growing areas to steep forest-covered hills. Part of the basin is covered with broad-leaved forest or bush; the main species, teak, is deciduous with a brief leafless period before the monsoon. The forest is especially thick in the hillier southeast where it is also wettest. Some forest is also distributed over the basin, apart from the clay plains. However, clearance for cultivation has had a considerable effect on its extent. At present about a quarter of the basin contains vegetation ranging from thick forest to scattered bush. The greater part of the basin is cultivated in the post-monsoon or winter season, with wheat and gram as the main crops. Some millet is grown during the monsoon season. Only about % of the whole area is irrigated at present. The existing hydrological network was well suited to a general study of the water balance of the whole basin. There was a good network of long term rainfall stations, reasonable meteorological records for Penman estimates of evaporation, and long term information on the inflow to the reservoirs at Dhukwan at the lower end of the basin or at Matatila just upstream.

4 The water balance of the Betwa basin RAINFALL In order to study the distribution of rainfall, the year period - was chosen for analysis. For those stations with a complete record covering the full period, monthly and annual averages were computed; there were in fact such stations within the basin and others nearby. Corresponding long term averages were computed for stations with a shorter record by comparison of ratios during the common periods with adjacent long term stations. Figure shows isohyets of the average annual rainfall distribution, PICHORv. DHUKVWN Downloaded by [...] at : anuary GUNA * Fig,, Scale Annual average rainfall - (in mm). km and it can be seen that there is considerable variation in rainfall over the project area. The heaviest rainfall of over mm occurs in the southeastern part of the basin, which is hilly and forested, while rainfall of less than mm is recorded in the north of the basin. The mean annual rainfall derived from the long term records for the whole Betwa basin above Dhukwan is mm. The monthly mean rainfalls at each station have been deduced from the records and are presented in Table. The average seasonal distribution is extremely similar over the whole basin, although the annual total increases from north to south. Table also shows the variability of annual rainfall as given by the coefficient of

5 .V.Sutcliffeefa/. Table Rainfall statistics Station Period of record Lat. Long. Elev (m) Monthly i mean rainfall imm) F M A M A S O N D Annual average (mm) Adjusted annual average (mm) CV Downloaded by [...] at : anuary Pichor Dhukwan Chanderi Lalitpur Pachhar Mungaoli Sironj Korwai Khurai Basoda Berasia Vidisha Bhopal (Bairagarh) Raisen Goharganj Begamganj Ghairatgan ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' variation (CV), the standard deviation divided by the mean. The figures deduced from the long term stations show that there is little change in CV over the basin, suggesting that the standard deviation is proportional to the mean. The uniform seasonal and annual variations make it simple to deduce the mean basin rainfall in an individual month or year. The rainfall at each station operating within the basin is expressed as a percentage of its own long term station mean; the mean of these percentages is taken as an index of the basin rainfall for the month or year; and this index is multiplied by the mean annual rainfall for the basin. For example, the monthly and annual rainfall totals for the whole basin for the period - have been deduced using stations within the basin and Lalitpur just outside to compensate for the short record at Chanderi. The results are given in Table and provide a long term series of rainfall over an area rather than at a point. This series, which is less likely than a single station record to suffer from errors of measurement, provides a useful basis for analysis of seasonal distribution, variability and trends. The seasonal distribution of rainfall is well defined by the individual monthly totals and the monthly averages. On average % of the total rainfall occurs in uly and August alone, while % occurs in the four months une-september. Heavy falls of rain are sometimes observed in the period October-December, while the lighter rainfall in anuary is more consistent. It is noteworthy that the CV of rainfall over the basin as a whole is.%, while the CV at individual long term stations ranges from. to.%. This illustrates the way in which rainfall over an area is much less variable than rainfall at a point. The possible existence of rainfall trends can be tested more reliably on an areal rainfall series rather than on point records.

6 The water balance of the Betwa basin Table Monthly rainfall series for the whole basin and runoff to Dhukwan, - Rainfall (mm) _ Runoff Year F M A M A S O N D Total (mm) Downloaded by [...] at : anuary Mean SD CV It is interesting to split the year record into five decades, for which the rainfall means are,,, and mm respectively. There is no indication of a secular trend in these figures. Persistence in rainfall from year to year may be revealed by serial correlation; this year record shows a lag-one serial correlation coefficient of., which is not significantly

7 .V.Sutcliffeef s/. different from zero. EVAPORATION Downloaded by [...] at : anuary The potential evaporation from a basin can be estimated from standard meteorological records by the Penman method, which combines an energy balance equation with an empirical term involving wind speed and vapour pressure deficit. The method uses standard observations of temperature, humidity, wind speed and radiation measurements or sunshine hours. The method depends on reasonable measurements or estimates of shortwave radiation received at the surface, and of the albedo or shortwave reflection. Estimates for mean monthly open water evaporation (E ) and potential evaporation (Eip) were made for Bhopal using albedo figures of. and., as recommended by Penman. The most direct estimates of incoming energy in the general area were provided by radiation measurements at Nagpur, about km southeast of Bhopal, where sunshine records are similar to those at Bhopal. Nagpur radiation data were used while Bhopal sunshine records were used for the back radiation term. Local temperature and wind speed measurements were taken from - averages. The estimates given in Table represent average evaporation. Because evaporation Table Open water evaporation (Eo) and potential transpiration (Ey) (in mm) estimated by the Penman method (Plinston & Hill, ). (Bhopal, ', \ m) O - LU LU F M A M A S is much more conservative than other water balance components, the Bhopal estimate may be taken to represent the basin and average monthly figures are preferred to estimates for individual months. N D Annual RUNOFF The long term annual total inflows to Dhukwan and Matatila dams have been converted to millimetres over the basin of km, and these data are included in Table. The mean annual runoff is equivalent to mm over the basin, with a standard deviation of mm and a CV of.%. As expected, the runoff varies much more than rainfall about the mean, because runoff is the residual of rainfall over evaporation. WATER BAUNCE The annual water balance of the Betwa basin may be simplified by considering the seasonal cycle as a single period of water surplus during the monsoon and a single period of deficit during the

8 The water balance of the Betwa basin remainder of the year. During the monsoon period, monthly rainfall is greater than potential evaporation for between two and four successive months, whereas during the rest of the year potential evaporation exceeds rainfall except in the occasional month. For the average year Fig. shows the period of surplus or excess of rainfall over evaporation, and the subsequent period of deficit. Downloaded by [...] at : anuary Fig. Schematic diagram of annual cycle. The soil moisture recharge may be superimposed on this diagram, which then shows, to use the terminology of Thornthwaite (), a period of soil moisture recharge followed by a period of water surplus, and then after the end of the monsoon a period of soil moisture utilization followed by a period of water deficit. The surplus includes groundwater recharge as well as surface runoff. The soil moisture recharge clearly must equal the soil moisture utilization; in the Betwa basin, because the soil moisture storage is generally full at the end of the monsoon and always reaches wilting point during the dry season, the annual recharge will bring the storage from wilting point to field capacity throughout the rooting depth. This difference is defined as the "root constant". The soil moisture recharge should be reasonably constant from year to year in the absence of land use change and may be considered as a first charge on the net rainfall, the surplus of rainfall over evaporation. Furthermore, brief periods of drought during the monsoon, and most periods of rain during the rest of the year, do not affect this cycle. The soil moisture deficit during dry periods in the monsoon will not approach wilting point, and the occasional rainfall surplus during November or December will not normally be sufficient to eliminate the soil moisture deficit. Thus, the net rainfall, or gross seasonal surplus, can be deduced by subtracting monthly potential evaporation from the monthly basin rainfall, and adding these monthly figures to give the net rainfall for the year. This calculation will be affected by the use of calendar months and basin rainfall. Because the beginning and end of the monsoon do not coincide with calendar months, some storms during a generally

9 .V.Sutcliffeefa/. Downloaded by [...] at : anuary dry month will be neglected. In months when rainfall exceeds evaporation at some but not all stations, the use of basin mean rainfall may also reduce the apparent surplus, but the widespread nature of the rainfall will make such months rare. The estimates of net rainfall might be improved by using lo-day totals, but these are not readily available; the use of monthly data may introduce a scatter to the data and perhaps reduce the apparent net rainfall in some years. If one accepts the conclusion that soil moisture recharge is a fixed charge on the net rainfall, and also makes the reasonable assumption that the surplus is then divided always in the same proportion between runoff and groundwater recharge, it is possible to make deductions about both processes by comparing seasonal net rainfall with annual runoff as indicated in Fig.. The soil moisture recharge is the intercept on the horizontal axis, or the net rainfall required before any runoff occurs; the groundwater recharge is the divergence between the line and the net rainfall/runoff points. Fig. Seasonal net rainfall, mm APPLICATION TO THE BETWA BASIN Groundwater recharge Schematic comparison of runoff with seasonal net rainfall. The approach is most directly applicable to a basin whose land use is reasonably homogeneous because the rooting depth and thus the soil moisture recharge and actual evaporation should be uniform. In the case of the Betwa a portion of the basin is covered with forest or with scrub and perennial grass. This vegetation appears to continue to transpire into the dry season and the source of this extra moisture must be a deeper root system. When considering the water balance of a whole basin, it is necessary to neglect such differences. The rainfall figures for the whole basin for the period - and the runoff records for the same period were compared. Figure shows the seasonal net rainfall, neglecting as irrelevant the occasional heavy rainfall in November and December, compared with the runoff expressed in millimetres from the whole basin. A line has been fitted by eye, ignoring the and outliers, in

10 The water balance of the Betwa basin -, - O - C < - Downloaded by [...] at : anuary Seasonal net rainfall, mm Fig. Seasonal net rainfall and annual runoff, -. order to estimate the intercept, as shown in Fig.. This shows that soil moisture recharge over the basin is about mm; the line is shown and suggests that the groundwater recharge in an average year is about mm. It should be noted that the groundwater recharge which returns to the river and emerges as runoff during the dry season is not revealed by an analysis of the annual runoff, but can be deduced by study of the monthly flows; this "shallow" groundwater storage at the end of the monsoon may be equated with the subsequent baseflow component of river flow. Flow records suggest that this baseflow component, or the runoff after the last month of surplus rainfall, is of the order of - mm over the basin. This analysis may be extended to other sites. Groundwater recharge is either discharged to the river during the dry season or if it is sufficiently deep is lost entirely to the gauging record; in other words recharge to surface layers or to deeper aquifers can be deduced from an analysis of the flow recession curve or from water balance studies respectively. The soil moisture recharge and storage, on the other hand, may be estimated simply by comparing net rainfall and runoff. DISCUSSION The analysis described has shown that the seasonal rainfall pattern is similar over the entire basin, that the standard deviation of annual rainfall is proportional to the mean, and a year series of monthly basin rainfall is presented. The potential evaporation during the monsoon months can be estimated with sufficient precision to give a reasonable estimate of the seasonal surplus or net rainfall. Gross and net rainfall may be compared with annual runoff for the same year period to give the following summary of the water balance: Mean Standard deviation CV Gross rainfall. Net rainfall. Runoff.

11 .V.Sutcliffeefa/. Comparison of annual values of the seasonal surplus and runoff shows that the seasonal soil moisture recharge can be estimated at about mm. This figure has a number of practical implications. It is the average amount of storage available over the basin for flood attenuation, and because its recharge is the first call on the rainfall this constant storage is available for crop growth after the monsoon each year, supplemented by limited rainfall during the growing season. Although an estimate of the groundwater recharge of about SO mm over the basin can in principle be deduced from the water balance, this estimate is in this case small compared with the scatter of the data. Physical studies are required to measure more precisely the groundwater recharge which takes place either locally to maintain dry weather flows or to deeper water bodies. Downloaded by [...] at : anuary ACKNOWLEDGEMENTS The authors are grateful to their colleagues in the Betwa Groundwater Project team for help in collecting records and for useful discussions. They are grateful to the India Meteorological Department for assistance. This paper is published by permission of the Chairman, Central Ground Water Board, New Delhi, and of the Director, Institute of Hydrology, Wallingford. REFERENCES Plinston, D. T. & Hill, A. () A system for the quality control and processing of streamflow, rainfall and evaporation data. Institute of Hydrology Report no., Wallingford, Oxfordshire. Thornthwaite, C. W. () An approach toward a rational classification of climate. Geogr. Rev., -. Revised paper received December