HYDROGEOLOGICAL INVESTIGATION IN A WATER SHED- A REPORT

Introduction:

A systematic hydrogeological study was carried out in the project area covering about…6500…ha. The survey includes geological and geomorphological mapping, periodic monitoring of water levels in a network of observation wells. 100% dugwells in the area have been selected as monitoring stations. Lithological log upto a depth of 10m (bgl) were obtained in certain areas to determine the geometry of the aquifers. Geomorphological and structural mapping was carried out in a larger area to determine the regional geology and trends of lineaments.

Complete geological mapping has been done on this area. The geology of the area is it is a granitic region with minor variation in the grain size. There is a pegmatite outcrop in the Sarenga Fulberia. There are basic intrusions in granite, which has a foliation dip and strike.

Geomorphologically the area can be subdivided as:

1. Denudation terraces

2. Shallow buried pediments

3. Moderately buried pediments

4. Valley fill deposits

The above four categories of geomorphological surfaces have been separately dealt with in the hydrogeological investigations as follows.

1. Pumping tests : Pumping tests to determine the aquifer parameters were carried out in a number of dugwells in the area. The wells were essentially situated in the Baid and Kanali area. The recuperation test data were analysed in G.W.W. programme and Kd (Transmissivity) value was determined.

2. Infiltration experiment: Infiltration experiments were carried out in the project area for determination of infiltration capacity. The infiltration capacity values are determined to classify the watershed in terms of infiltration capacity.

3. Monitoring of water level: From 160 nos. observation wells of water table below ground surface has been monitored every month. From these data pre-monsoon and post- monsoon depth to water level maps and water level fluctuation maps have been prepared.

4. Reduced level connection of wells: All wells have been connected by land level survey and reduced level (RL) of each well was determined. Thus RL of water table at each well point in each season was computed. Seasonal water table contour maps have been prepa- red to determine the groundwater flow direction and the geometry of the saturated surface.

5. Stream discharge measurements : At the mouth of the Chaggalkuta stream gauging station has been set up to measure the stream discharge continuously.

6. Meteorological Observations: Primary meteorological data have been generated at the weather station set up at Teghori village. Daily precipitation, sunshine, wind speed, rate of evaporation, maximum and minimum temperature, humidity etc. are continuously recorded at this station. With the help of necessary meteorological data daily PET (Pennman) has been computed with the help of a programme prepared for this purpose.

7. Soil Moisture: Monthly soil moisture percentage has been gauged at every land situation at 15 cm and 30 cm depth. With this data monthly change in soil moisture has been determined.

Aquifer Condition: The basement rock of Chagalkutta watershed is mainly granite along with some pegmatite and basic intrusive. Fractures in this rock system generated the secondary porosity. The weathered residuum and the thin alluvium cover that lie over the basement rock contain water and these along with the fractured part of the basement rock developed the treatic aquifer system. This area has a single unconfined aquifer and thickness is between 15 to 30m as revealed from local information of dug wells and hand tube wells. But these tube wells and dug wells are mostly confined to the ridge, back slope and rarely to the toe slope and valley region. There is hardly any well in the valley fills.

212 (100%) dug wells are monitored every month. It is very difficult to take water level from the drinking water tube wells because the local people do not allow opening of those hand pumps. So the present observations are totally based on the data available from those 212 wells.

It is observed that the wells are in most cases occur as clusters in settlement areas in some cases spacing between two wells is very small to generate contour lines between them. So 50 wells have randomly been selected for water level map generation water table contour maps have also been generated by RL connection data of those wells.

All contour maps and depth to water level zonation maps have been prepared with the help of SURFER-6 software and for better interpolation and extrapolation kriging method has been adopted for grid generation. Each depth to water level grid file and each contour grid file have been blanked with watershed.bln.

Depth to water level condition: Pre-monsoon and post-monsoon depth to water level maps have been prepared using the table (well.xls). The map shows that in the chhagalkuta watershed there is wide variation in the pre-monsoon water level (April 1999). The maximum depth to water level condition has been observed at Lakhanpur (well no. Lak 1) where the water level is 14.30 mbgl. At Golghori wide variation in water level is observed (5.05m to 11.05m) bgl. At Siromanipur, Danga, Kurchibedia, Kasibedia, etc. area water level is between 8 to 9m bgl. But it is observed that in most of the wells other than those described above major area of the watershed has depth to water level between 6 to 7m bgl. At Gopaldihi, Panjangora, one well at Kendsar (ken 1) shows water level below 5m and 2 wells at Gopaldih (Gop 1 and Gop 5) show water level within 4m bgl. The depth to water level map divided the watershed in depth to water level zones.

1) Between 3-6m bgl i.e. within centrifugal pumping limit

2) From 6-7m bgl i.e. sump well is possible

3) From 7-10m bgl i.e. beyond centrifugal pumping limit but within low HP submersible

Pumping limit.

4) Above 10m bgl i.e. special pumping device or high HP submersible pump is required.

Post monsoon depth to water level map (November) shows a remarkable rise in water level. It is to be mentioned that at Lakhanpur the rise is maximum (11 to 12m approximately). During the post monsoon period major part of the watershed show depth to water level between 2 to 3m bgl. Only at Kasibedia, Poirasol, Kanudih and Ghutia. Post-monsoon water level is between 3 and 5.5m. In the western ridge area i.e. at Dangra Lari and Kendsar and Panjangora in the south, the fluctuation is minimum (75m). There are some isolated patches showing fluctuation between 7 and 9m (at Danga).

Groundwater flow direction

Water table contour maps generated during pre and post monsoon period show the major trend in groundwater flow. Pre-monsoon water table rises upto 160m amsl at the southwestern corner of the watershed and comes to 99m amsl at the month of the Chhagalkota stream. The overall groundwater flow direction is from southwest to northeast in conformity with the surface.

During the post-monsoon period overall flow direction does not change much excepting with a rise in level. In the southwestern corner the R.L. of water table is about 165m amsl. In the northernmost corner of the watershed i.e. near the mouth of Chhagalkuta this value is about 103m amsl.

. HYDROLOGICAL ANALYSIS:

1. Purpose and scope:

Hydrological analysis carried out in Chagalkuta watershed started from February 1999. All investigations were conducted from the field station situated at Teghari in Chhatna block of Bankura district. The field station is equipped with all types of meteorological and hydrological gauging provisions

2. Parameters:

For detailed hydrological analysis the following parameters had been analysed in details.

i) Precipitation

ii) Potential evapotranspiration

iii) Soil moisture

iv) Pond water volume

v) Groundwater dynamic storage

vi) Runoff

vii) Groundwater draft

viii) Actual evapotranspiration

3. Basic data Generation:

For calculation of each of the above-mentioned parameters primary data have been generated from field and then computed. The table below shows the list of field database and frequency of reading behind each of the parameters mentioned above.

Table showing List of field database behind each hydrological parameter

Sl No.	Parameter	Field Data base	Reading frequency	Remarks
1	Potential evapotranspiration	Sunshine hour	Daily
		Wind speed
		Mean air temperature
		Relative humidity
2	Precipitation	Rain gauge reading	Daily
3	Soil moisture	Soil moisture %	Monthly
4	Pond water volume	Field measurement from pond	Monthly
5	Evaporation from pond	Pond surface area	Monthly
6	Groundwater dynamic storage	Depth to water level	Monthly
7	Runoff	Measurement from stream	Daily
8	Groundwater draft	Human use / irrigation draft/	Monthly
9	Actual evapotranspiration	Crop area for each crop	Monthly

4. Method of data collection.

The methods of collection of different types of field data are described below.

I. Daily sunshine hour is recorded from sunshine recorder installed at Teghai field station.

II. Daily maximum and minimum temperature is recorded using maximum-minimum thermometer and Stevenson’s screen. Mean daily air temperature is calculated from the readings.

III. Daily relative humidity is collected using hygrometer.

IV. Anemometer is used to collect the wind velocity. Daily two readings are collected and daily average wind speed is calculated.

V. Daily reading from rain gauge is collected at the hydro-meteorological station at Teghari..

VI. Daily evaporation rate is measured using pan evaporimeter.

VII. There are dug wells within the watershed area. Depth of water level from ground level is measured from each well every month.

VIII. The wet area and depth of each pond is measured every month to calculate the monthly pond volume.

IX. There is a gauging station at the confluence of Chagalkutta with Arkosa. At that point the daily stream flow is measured using flow meter.

X. The population of each mouza is an indicator of groundwater draft. In this area people depend mostly on ground water for their domestic use. Multiplying the population with the daily water use and the number of days in a month the groundwater draft is calculated.

XI. Soil moisture data are collected from the field using soil moisture meter, which gives the percentage of available water.

XII. Area of each crop is recorded in every month.

5. Computation

a) Computation of Potential evapotranspiration: this is calculated using Penman’s equation:-

PET= (AH_n + Eag)/(A+g)

Where PET = daily potential evapotranspiration in mm per day

A= slope of the saturated vapour pressure vs. temperature curve at the mean air temperature, in mm of mercury per ^oC. This value is achieved from the Table M1

Hn = net radiation in mm of evaporable water per day

Ea = parameter including wind velocity and saturtion deficit

g = psychrometric constant = 0.49 mm of mercury / ^oC

The net radiation is expressed as Hn= Ha (1-r) [a+bn/N] -st⁴a ( 0.56-0.092√e_a )[0.10+0.90n/N]

Where Ha= incident solar radiation outside the atmosphere on a horizontal surface, expressed in mm of evaporable water per day ( it is a function of latitude and the period of the year as computed in table M2)

a = a constant depending upon the latitude Φ and is given by a= 0.29 cos Φ

b = a constant with an average value of 0.52

n = actual duration of bright sunlight in hour

N = maximum possible hour of bright sunshine (it is a function of latitude as indicated in table M3)

r = soil albedo

s = Stefan- Boltzman constant= 2.01 X 10^-9 mm/day

Ta = mean air temperature in degrees Kelvin = 273 + ^oC

e_a = actual mean vapour pressure in the air in mm of mercury

The parameter Ea is estimated as Ea = 0.35 [1+ u₂/160] (e_w-e_a] in which

u_{2 =} mean wind speed at 2m above ground in km/day

e_w = saturation vapour pressure at mean air temperature in mm of mercury this is defined as a function of temperature and the equation is = 4.584 exp [17.27t/ ( 273.3+t)]

e_{a =} actual vapour pressure

For computation of PET the following basic data are necessary:- Latitude, Elevation, Mean temperature, Mean relative humidity, Observed sunshine hour, Wind velocity at 2m height, Nature of surface cover, Date of observation.

b) Computation of Soil Moisture volume: Soil moisture was monitored periodically. Soil moisture percentage data have been gauged from every mouza and every land situation in every month. For computation of monthly water balance, monthly average soil moisture percentage for every land situation was calculated first. From monthly soil moisture percentage soil depth of each land situation and soil character, soil moisture depth and volume have been computed. The following conditions hereby apply. For determining field capacity and wilting point a graph (fig 3.8) of page 65 from the book Hydrology and Management of Watershed by Keneth. N. Brookes, Peter F. Ffolliott , Haus. M Gregersen and Leonardo F Dabauo have been consulted. The table shows the average root depth, field capacity, wilting point and soil water holding capacity.

Soil water holding capacity (mm)=Root zone soil depth (m) x {Field capacity (mm) –Wilting point (mm)}

Land situation	Average Root Zone Soil Depth (m)	Field capacity (mm)	Wilting point (mm)	Soil moisture holding capacity (mm)
Ridge	0.15	120	30	13.5
Back Slope	0.45	240	60	80
Toe Slope	0.80	200	40	128
Valley Fill	0.90	370	150	198

From soil moisture % soil moisture depth of each land situation for each month has been calculated.

Soil moisture volume (in ham) was calculated as = Soil moisture percentage x Soil moisture holding capacity (in mm) x area (in ha) x .001

c) Computation of Groundwater recharge/ discharge: Ground water level (depth to water level) has been measured in every well in every month. Water level fluctuation of each well for each month with respect to previous month was calculated.

From mouzawise average fluctuation groundwater recharge and discharge have been calculated using the equation; Recharge / Discharge = Fluctuation x Area x Specific Yield

Since the total area falls within a granitic country and the aquifer mainly comprises of weathered and fractured granite the specific yield has been assumed as 0.05.

d) Computation of Pond volume:

Actual pond water volume of every month was measured (average depth x area) for every pond. Monthly total pond water storage was calculated from that value.

e) Computation of Actual Evapotranspiration:

Actual evapotranspiration (AET) has been calculated from the potential evapotranspiration (PET) using the following equation :

ET (vol ) in ham = PET (mm) x S Crop & other vegetation Area (ha ) x Crop Coefficient (k_c) for each crop x (0. 001)

f) Computation of evaporation from surface water bodies: Evaporation from surface water bodies is calculated using the following equation.

Evaporation in a month = pond surface area (Ha)* evaporation from pan evaporimeter (mm)* pan factor

In this case the pan factor is 0.70

WATER BALANCE:

The fundamental water balance equation used in this project is a modification of the Thornthwith Mather model in which groundwater recharge/ discharge and groundwater draft have been included. Thus the principal water balance equation is

P + surplus water of previous month + I = AET+ D Sm. + DGW + D Pond + evaporation + GW Draft + Runoff + Surplus water retained in the system at the end of the month.

Here P= precipitation volume

I = Irrigation input

AET = actual evapotranspiration

D Sm = change in soil moisture volume

DGW = Change in groundwater storage (recharge/ discharge)

D Pond = Change in pond storage

Table Showing the parameters and their mathematical relations involved in calculating the water balance

Item No	Parameter	Algorithm
A	WATER RETAINED IN THE SYSTEM	Water retained in the system at the end of previous month (+ve value=surplus, -ve value = deficit)
B	PRECIPITATION VOLUME (P)	Precipitation measured for the month * area of the watershed
C	IRRIGATION INPUT(I)	Actual irrigation input in the field, calculated on the basis of irrigated crop pattern and area
D	TOTAL WATER INPUT	Total of Water retained in the system at the end of previous month+ precipitation volume+ Irrigation input
E	SOIL MOISTURE CHANGE	Soil moisture volume of previous month - soil moisture volume of current month
F	POND VOLUME CHANGE	Pond volume in the previous month – pond volume in the current month
G	GROUNDWATER RECHARGE/ DISCHARGE	Average change in groundwater level * area of watershed* specific yield [+ve value means recharge into, -ve value means discharge from the reserve]
H	WATER GAIN IN THE RESERVES	Total of Abstraction of water in reserves= soil moisture change+ pond volume change+ groundwater recharge/ discharge
I	AET	Actual evapotranspiration based on Crop area* crop coefficient*PET
J	EVAP	Evaporation from wetlands
K	RUNOFF	Defined
L	TOTAL WATER LOSS	Total of AET+EVAP+RUNOFF
M	WATER RETAINED IN THE SYSTEM AT THE END OF THE MONTH	Total water input-Total abstraction of water in reserves-Total water loss[+ve value means water surplus and -ve value means water deficit ]

6. Water Estimation:

Water estimation is done to achieve at a conclusion that for a particular time and space there must be a tool for assessing the available water at any part of the whole system.

Thus the whole system is divided into three parts, i) water gaining system, ii) Water storage system and the iii) discharging system (water loss)

Water gain of the system is Precipitation, Irrigation (in case of the Chagalkutta watershed it is nil as no irrigation is being received from outside), and cumulative change of soil moisture storage, change in pond water volume, change in ground water volume. Total water loss from the system occurs as Evaporation loss, Actual ET, ground water seepage loss, and run off. The difference in water gain and water loss is the quantum of water that remains impounded within the system through various water harvesting structures excluding the traditional pond/tank system. It was found that some surplus water is generated within the system. It is explained that a huge amount of water is kept stored in the field (impounded) by the local people from the very first month of the monsoon to minimize runoff. But, how much water is stored cannot be quantified. Soil moisture is also measured from comparatively dry areas; therefore the surplus water is actually the artificial and temporary impoundment, which subsequently drains into streams or groundwater. So, for rational calculation the surplus water found in a month is added to the total water input of the next month.