GROUNDWATER EXTRACTION THROUGH AGRO-WELLS AND ITS … · Department of Geography, University of...

International Journal of Scientific Research and Innovative Technology ISSN: 2313-3759 Vol. 4 No. 6; June 2017

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GROUNDWATER EXTRACTION THROUGH AGRO-WELLS AND ITS IMPACT

ON GROUNDWATER AVAILABILITY OF TANK CASCADES IN THE DRY

SEASON: A CASE STUDY IN THE DRY ZONE OF SRI LANKA

Dr. Muditha Prasannajith Perera

Senior Lecturer,

Department of Geography, University of Peradeniya, Peradeniya, Sri Lanka.

[email protected], [email protected]

Abstract

From the 1950s, a number of scientists have investigated the possibility of introducing Agro-wells to

use the shallow ground water, of the dry zone of Sri Lanka during the dry season. However, the rate of

construction of Agro-wells has been accelerated with the intervention of the Agricultural Development

Authority and the Provincial Councils since 1989. Then onwards the diffusion of Agro-wells has been very

rapid. The average water extraction from a single well for a season was approximately 2100 m3. With the

expansion of Agro-well irrigation, some scientists had been warning that water extraction through Agro-wells

might be a serious hazard to the groundwater availability in the dry season. Therefore the current study was

launched to investigate whether there is a significant impact of water extraction through Agro-wells on

groundwater availability in the dry season. It was a comparative case study between a high Agro-well density

cascade and a low Agro-well density cascade. The strategy of examining the status of groundwater

availability was by conducting the “Half Recovery Pumping Tests” (T1/2) and computing the “Well Specific

Capacity” (k). The results revealed that during the cropping period, there was a considerable reduction of

groundwater availability in high-Agro-well density cascades when compare to low Agro-well density cascade.

But the difference of groundwater reduction during the dry season was not at a significant level, according to

differentiate significant test.

Key Words: Agro-wells, Shallow Groundwater, Groundwater Extraction, Tank Cascades,

Half Recovery Time, Well Specific Capacity.

.


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1. Introduction

In the 1950s, Farmer (1951) explored the potential of groundwater in the dry zone of Sri Lanka,

considering the geological similarity of South India, and Sirimanna (1952) studied the groundwater

availability in the hard rock regions of this country. Later Panabokke (1959) , Fernando (1973) Madduma

Bandara (1973, 1977), and Dharmasena (1998) revealed the ground water behavior in the dry zone and the

possibility of using groundwater for agriculture.

The Anuradhapura Dry Zone Agricultural Project (ADZAP) was implemented over a five year period

from 1981, to the establishment of a well developed farming system in the project area, including the

restoration of minor tanks and practicing and encouraging Agricultural wells or Agro-wells (Jayasena,1991).

However, the rate of construction of Agro-wells to use shallow groundwater has accelerated with the

interventions of the Agricultural Development Authority (ADA) and the Provincial Councils since 1989

(Pathmarajah, 2002). “The National Agro-well Programme” was the key intervention. In addition, various

nongovernmental organizations such as the International fund for Agricultural Development (IFAD), Asian

Development Bank (ADB), and a few Non Governmental Organizations including Isuru Foundation also

extend subsidies and subsidized loans for the construction of Agro-wells (Kikuchi et al. 2003). Consequently

the number of Agro-wells in Sri Lanka has been increased approximately up to 120,000 while being the

number of Agro-wells in Anuradhapura district as 19,600 (Perera, 2016).

The topography of the central dry zone consisted of a thin weathered soil zone which appears to be

overlaid by a thin alluvium layer in the lower and middle parts of the small valleys that consist of small tanks.

This weathered bedrock is the aquifer which serves as the groundwater reserves for Agro-well irrigation

(Dharmasena and Goodwill, 1999). One of the realistic facts is that, the construction of small tank systems in

the dry zone was one of the major efforts to maintain the ground water level closer to the land surface

(Dharmesena, 2002). Further, Panabokke (2002), and Senaratna’s (2002) explanations, revealed that the tank

cascade acts as a “Physical Unit” with natural and human activities.

Dharmasena and Goodwill (1999), also explained about this physical unit as the generally muted

topography of tank cascades in the dry zone yields a thin weathered zone. Madduma Bandara, (1985) and

Panabokke et al. (2002) further explained this as follows, “these cascade patterns are located within mini

basins with second order or first order ephemeral streams”. However, it was clear that these mini basins are

hydrological units and their available water could be used for the benefit of the people as well as natural

processes.

Sakthivadivel et.al. (1997) attempted to develop criteria to assess the cascade water availability in the

North Central Dry Zone. The criterion was the “Initial Cascade Screening” (ICS) to grasp the basic cascade

hydrologic data. Somasiri (1979) and Dharmasena (1989) have developed the tank water balance

methodologies considering the input and outputs. Itakura, (1995) developed a water balance model for

planning rehabilitation of a tank cascade irrigation system. Further, Jayatilaka et al. (1997) prepared a model

to predict the water availability in irrigation tank cascades and Matsuno et al. (2003) conducted a study on

return flows in a cascade system. Perera, Nianthi, and Madduma Bandara (2016) made groundwater table

contour maps for selected tank cascades in the Dry Zone of Sri Lanka and they found a fluctuation pattern

within a climatic year.

A study was carried out by Dharmasena (1994) to identify an indicator to reflect the recharging ability

of a shallow dug well in an aquifer. The index was termed as the "Half Recovery Time" (T1/2) and it has been


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derived from basic relationships of well hydrology. It was defined as the time taken to recover half the depth

pumped out from a well. Most of these past studies on water availability in tank cascades were carried out

only for the irrigation command area requirement or tank rehabilitation processes. Only very limited attempts

were made to study the ground water availability for Agro-well development such as in Dharmasena’s study.

If the high water available cascades are extremely vulnerable to the development of Agro-wells, it

would create a solution. And also if the available water is exploited by the Agro-well water extraction, there

will be a serious impact to all hydro-ecological needs as well as basic human needs in the tank cascade.

Further, Perera (2016) revealed that, Dry Zone farmers also believe that there are hydro-ecological impacts in

tank cascades due to Agro-well development.

De Silva (1998) also explained about the general background of the groundwater condition in the dry

zone of Sri Lanka as “90% of the dry zone areas of Sri Lanka are covered by metamorphic crystalline rocks,

called ‘hard rocks’, therefore the ground water potential in the dry and intermediate zones is comparatively

limited due to low storage and transmissivity”. Further, Dharmesena, (2002) revealed that the groundwater

potential in the dry and intermediate zone is comparatively limited. They have argued that un-controlled Agro-

well development will damage the ground water availability. As there is a limited amount of available ground

water especially in the dry season, there should be a limited number of Agro-wells that can be successfully

operated in the dry period. Most of the dry zone farmers use Agro-well water from April to August. This is

considered to be one of the most suitable time frames for cropping although this period lacks surface irrigation

water. This situation causes the ground water availability to decrease (Dharmasena, 1998) . Panabokke (2002)

also revealed that, depletion of the groundwater due to over extraction may be a serious hazard in tank

cascades. This situation is further clarified by Dharmasena. (2002) as, the shallow aquifers in the local valley

alluvium and the exploitation of ground water, using Agro-wells in some micro catchments in the dry zone,

may lead to a net depletion of groundwater. All these evidence warn that the increase of groundwater

extraction through more Agro-wells may seriously decrease the groundwater availability in tank cascades.

Accordingly, this study was conducted to investigate the impact on groundwater availability in the dry season

due to Agro-well development in tank cascades, using the Dharmasena’s Half Recovery Time (T1/2) and

computing the Well Specific Capacity.

The hypothesis of the current test was “Groundwater extraction through Agro-wells significantly

reduces the groundwater availability in tank cascades during the dry season”.

2. Methodology

The method was to compare the status of the groundwater availability between high Agro-well density

cascades and low Agro-well density cascades during the Agro-well based agricultural season (May to

September). For this purpose, Dharmasena’s strategy of “Half Recovery Pumping Tests” and computing the

“Well Specific Capacity” for both types of cascades were used.

The Half Recovery Time (T1/2) can be given by:

T1/2 = 0.54 D2

K

Where, D = well diameter, K = well specific capacity, 0.54= constant value of the aquifer

characteristics for the cascade environments.


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The well specific capacity has been developed using the same formula as;

Where, D = well diameter, T1/2 = half recovery time and 0.54= constant value of the aquifer

characteristics for the cascade environments.

Background of selected two cascades

i. Equal depth to bedrock

ii. Equal slop percentage of both cascades

iii. Equal amount of groundwater extraction for domestic purposes

iv. Equal land use types

v. Very same cropping and water issue periods from small tanks.

vi. Only Agro-well density is different. That mean groundwater extraction is deferent

In addition to that, following assumptions were also made.

i. Equal pumping rates for each tests

ii. Rainfall occurring for both cascades during the test period (May to September 2013) is same.

iii. No rainfall occurring during the pumping test

iv. Equal porosity and permeability condition of the soil

v. Agro-well diameter of all wells were within a small range

vi. Equal range of elevation from tanks, to test wells.

Two tank cascades in upper Malwathu Oya basin, having the same resources characteristic except

the Agro-well density, were selected. (Periyakulama Cascade = Agro-well density is High (22 per sq

km) and Halmillawewa Cascade = Agro-well density is Low (5.4 per sq km))

K = 0.54 D2

T1/2


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Table: 1 Status of selected tank cascades

Nu. Category Halmillawewa tank

cascade

(A/w density - Low)

Periyakulama tank

cascade

(A/w density - High)

1 Residential families 123 135

2 Number of floral land

use types

07 07

3 Agro-ecological region DL 1(Dry Low lands) DL 1(Dry Low lands)

4 Mean annual rainfall 1250 – 1400 mm. 1250 – 1400 mm.

5 Depth to bed rock

(average)

6 m - 6.5 m 6 m - 6.5 m

6 Slope % (Average) 1 % 1 %

7 Soil grade Medawachchiya-Aluthwewa-

Divulwewa-Hurathgama-

Kahatagasdigiliya

Association (No-37)

Medawachchiya-Aluthwewa-

Divulwewa-Hurathgama-

Kahatagasdigiliya

Association (No-37)

8 Tube wells 01 01

9 Domestic wells 22 20

10 Agro-well density 5.4 Per/km2

22 Per/km2

Source: Agrarian service departmental documents and field study 2012

The average diameter of Agro-wells in the study area was 5.6 m and average depth was 7.3 m while

the average depth to bed rock was 6.4 m. The current study has shown that the average ground water level

fluctuates between 3.6 m – 6.9 m in the dry months (July – September) and about 90% of Agro-wells had at

least 2.0 m water depth in the most dry months. The average pumping hours from a well in both study

cascades was 5 hrs. Further average water pumping for an hour was 12 m3 and average water usage of one

Agro-well per day was 60 m3. Average range of days of water pumped out in season was 32-40 days.

Consequently, average water extraction from one well in the season was approximately equal to 2100 m3.

One of the key important points for this study explained by prof. Dahanayake by saying, “the

maximum depth of Agro-wells may be 8 to 9 meters, and these wells were constructed in alluvial deposits or

the “C” layer of the rock profile. This means not in the bed rock as “Tube wells”. Therefore when we study

the water behavior of Agro-wells, the more important thing is paying attention to the soil profile and related

shallow aquifers in valleys, than the geological background”.

This opinion revealed that, when we compare the different tank cascades, the geological background

will not be a significant considerable factor. Further, according to the Dharmasena’s view when selecting

samples for this comparative study, there should be similarities of the cascade characteristics including similar

depth to the bed rock, in the soil profile.


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Therefore, the similar “Soil Profile Zones” which contain a similar shallow groundwater situation were

identified. For this purpose, 60 cross sections of the unlined Agro-wells were checked and similar soil profile

zones were identified for selecting sample Agro-wells for the pumping test.

Figure: 1. A Sample soil profile of the study area

Source: Field study 2013

Then the 6 Agro-wells per cascade in similar soil zone, representing the upper, middle and lower parts

in the cascades were selected. According to a mutual understanding with farmers a ‘test date’, at least after 3

days of the final pumping, was selected. On the planned test day, water of 4 feet height was pumped out from

the well and time measured until 2 feet (Half) recovery.

Figure: 5.3 Pumping test Figure: 5.4 Water outflow

Source: Field study 2013 Source: Field study 2013


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The same test was done, three times within the Agro-well based cropping period

i. Beginning of the Agro-well based cropping period (May, 2013)

ii. Middle of the Agro-well based cropping period (July, 2013)

iii. End of the Agro-well based cropping period (September, 2013)

3. Results and Discussion

Half recovery times of both cascades were increased during May to September. It was a normal

situation due to high percolation and evaporation in addition to extraction. However, half recovery times have

doubled from May to September, in both cascades. Then the average half recovery times of separate cascades

were calculated. But a difference between Halmillawewa (A/w density 5.4 per Sq. km) and Periyakulama

(A/w density 22 per Sq. km) cascades could not be identified.

Then, the “well specific capacity (K)” was computed from half recovery time (Table 2). Further the

well specific capacities (K) relevant to the different periods during the Agro-well water extraction period were

separated and analyzed as follows.

i. Early dry period (May/ with the beginning of Agro-well water extraction period)

ii. Mid period (July/ in the middle of the Agro-well water extraction period)

iii. Late dry period (September/ with the end of Agro-well water extraction period)

The well specific capacity (K) from half recovery time.

T1/2 = 0.54 D2 = K = 0.54 D

2

K T1/2

(T1/2 = time taken to recover half the depth pumped out from a well, D = well diameter, 0.54 = constant

value of the aquifer characteristics for the cascade environments,

K = well specific capacity)

Table 2: Well specific capacity

No Sample Agro-

well

D

(Diameter)

(m)

D2 Testing month

(Mid of the

month, year

2013)

T ½

Half recovery time (Hours & Minuets)

K

K = 0.54 D2

T ½

(m2/hr)

1 H-1 5.6 31.3 May 4.00 4.2

July 6.10 2.8

Sep 8.20 2.0

2 H-2 5.4 29.1 May 2.30 6.8

July 3.15 4.9

Sep 5.15 3.1


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3 H-3 5.6 31.3 May 3.00 5.6

July 4.15 4.1

Sep 6.15 2.7

4 H-4 6.0 36.0 May 3.00 6.5

July 4.00 4.8

Sep 5.30 3.7

5 H-5 4.8 23.0 May 4.30 2.9

July 5.45 2.3

Sep 7.30 1.7

6 H-6 5.6 31.3 May 3.00 5.6

July 4.20 4.0

Sep 6.10 2.8

7 P-1 5.6 31.3 May 2.35 7.2

July 4.00 4.2

Sep 5.40 3.1

8 P-2 5.8 33.6 May 2.10 8.6

July 3.30 5.5

Sep 4.50 4.0

9 P-3 6.0 36.0 May 3.00 6.5

July 4.20 4.6

Sep 5.10 3.8

10 P-4 5.6 31.3 May 5.00 3.4

July 6.15 2.7

Sep 7.00 2.4

11 P-5 6.0 36.0 May 3.30 5.9

July 4.30 4.5

Sep 6.00 3.2

12 P-6 5.6 31.3 May 3.30 5.1

July 4.50 3.7

Sep 7.10 2.4



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Then, the well specific capacity (K) values (m2/hr) relevant to the different periods during the Agro-well water

extraction were compared with two cascades.

Graph: 5.4 Comparison of the well specific capacity (K)


Accordingly, it was revealed that the decreasing of the well specific capacity is comparatively high, in

the high Agro-well density cascade (Periyakulama), during the dry period (Agro-well based cropping period).

Therefore, a significant test was conducted to check whether the difference was significant or not.

Table 3: Difference mean T – Test of well specific capacity

Group Halmillawewa Periyakulama

Mean 2.600 2.967

N 6 6

P -value and statistical significance:

The two-tailed P value equals 0.5362

This difference was considered to be not statistically significant.


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When considering the above background, the following observations could be made;

i. Half Recovery Time of all sample Agro-wells in both cascades increased, during the Agro-well

cropping period (Dry period).

ii. Well specific capacity of all Agro-wells in both cascades were also reduced, during the Agro-well

based cropping period.

iii. The decrease of the well specific capacity during the Agro-well based cropping period, is

comparatively high, in the high Agro-well density cascade (Periyakulama).

iv. The difference of the reduction of well-specific capacity between two cascades was not significant

according to the significant test.

4. Conclusion

The major role of this groundwater availability experimental test was “half recovery time testing” and

calculation of the “well specific capacity”. During the study it was revealed that the half recovery times of

both cascades increased from May to September and there were not any outstanding differences. However, it

was revealed that the decrease of the well specific capacity was comparatively high, in the high Agro-well

density cascade (Periyakulama), during the Agro-well based cropping period. But this difference was not

statistically significant according to the significant test.

Accordingly, it was clear that the groundwater availability as well as the well specific capacity have

been reduced during the dry period in both types of tank cascades, while the Agro-well based agricultural

activities were in process. The reasons may be the “natural loss” of the ground water in the dry season due to

groundwater flows (according to the natural gradient), percolation, evaporation and transpiration.

Although there was an impact of groundwater extraction through Agro-wells as revealed by the well

specific capacity, it has not created a significant impact, at the current groundwater extraction conditions in the

area.

According to the above findings, the conceptual hypothesis of this test; “Groundwater extraction

through Agro-wells significantly reduces the groundwater availability in tank cascades during the dry season”,

could be rejected.

Acknowledgements:

Author gratefully acknowledges the financial supports given by the International Research Centre,

University of Peradeniya, Sri Lanka(Grants Reference Number: InRC/RG/13/06) and National Centre

for Advance Studies in Humanities and Social Sciences, Sri Lanka(Grants Reference

Number:14/NCAS/PDN/Geo/37).


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