5.7 Conjunctive use of surface and groundwater - Krishna Delta, India

Introduction

In cases where a river system has been exploited to the maximum extent, and where large, unmet demands for water continue to exist, it is often necessary to exploit groundwater resources wherever they exist. However, in the lower reaches of the system, it is also likely that seawater intrusion will reduce the potential exploitation of groundwater resources in the deltaic reaches of the system. In such situations, mixing of groundwater with surface water can be used to augment water supplies.

The Krishna River flows through the states of Maharashtra, Karnataka and Andhra Pradesh (AP), and has reached this state of maximum surface water exploitation. In accordance with the Bachawat Award for the distribution of river waters, Andhra Pradesh State has an allocation of 22653 million m³, most of which is currently being exploited or has been earmarked for exploitation in the foreseeable future, leaving some areas of Rayalasema without enough water to satisfy the minimum requirements. In view of this, the Andhra Pradesh State Government has commissioned the Telugu Ganga Project, a prerequisite for which is the conservation of the waters of the Krishna River upstream of Nagarjuna Sagar Reservoir. The use of groundwater in conjunction with surface water resources has been proposed as part of these conservation measures. According to information published by Andhra Pradesh State Irrigation Development Corporation (APSIDC), the utilizable groundwater resources of the Krishna Delta are 4 568 million m³, against an annual net utilization of 287 million m³. Hence there is enough scope for utilization of groundwater in the delta provided it is blended with available surface water to achieve an acceptable quality for, primarily, agricultural use.

Technical Description

In order to meet the minimum water requirements of the Delta area, the quality of groundwater and surface water in Krishna Delta ayacut was considered in order to determine an appropriate blend of groundwater and canal water to support their conjunctive use. The quality of groundwater along the coastline is generally poor due to the intrusion of seawater into the coastal aquifers. Chloride concentrations are generally found to be in the range of 85 to 845 ppm. For this reason, crop irrigation mainly depends on canal waters as the primary water source. Two methods of mixing surface and ground waters were investigated; namely, supplying surface and ground waters separately for alternate waterings in the required proportions, and directly blending surface and ground waters by pumping groundwater into the canals.

Extent of Use

APSIDC collected ground and surface water samples at Tenali, Duggirala, and Pedavadlapudi in the Krishna Western Division, at Nidumole, Gudur and Pamarru in the Krishna Central Division, and at Eluru, Gudlavalleru and Mudinepalli in the Krishna Eastern Division, and analysed the samples for Total Dissolved Solids (TDS) and chloride concentrations, hardness and Electrical Conductivity (EC). Selected results are shown in Table 38. These analyses, and related soil samples, collected at root zone depth, suggested that the conjunctive use of surface and ground waters was possible, as shown in Table 38, and that concerns regarding salination of the soils were unfounded. Insignificant concentrations of chlorides were present in the soil, probably due to leaching by rainwaters during the Khan/season. The import of this finding was that no allowance need be made for using additional canal waters to "sweeten" the soils by leaching excess salt from the soil profile.

TABLE 38. Groundwater Quality in the Krishna Delta^a.

^a In all the above samples, sulphate concentrations were found to be below the limit of detection, and the value of electrical conductivity was less than 1 000 microMhos/cm.

TABLE 39. Water Quality Standards for Irrigation Water.

The results of the analyses from the Krishna Western Division indicated dissolved solids concentrations in groundwater and canal water samples of 1 364 ppm and 597 ppm, respectively. Blending these waters in a 1:1 ratio by volume results in a final TDS of 981 ppm, which is below 1 000 ppm threshold considered as the minimum quality for irrigation water. Similarly, in the Krishna Eastern Division, a 1:1 ratio of groundwater and canal water was found to produce a product water that is suitable for irrigation use. In contrast, in the Krishna Central Division, blending groundwater and surface water in ratios of up to 34:1 failed to result in a product water with a TDS concentration of less than 1 000 ppm. Hence, the utilization of groundwater is this Division was not considered further. Finally, in the Krishna Delta ayacut system, a blend of groundwater and surface water in a ratio of 28:72 was found to produce a product water suitable for irrigation use.

Operation and Maintenance

The operation and maintenance requirements of this technology includes the inspection and repair of pipelines, channels, and pumps, all of which make use of existing skills present within the agricultural community.

Level of Involvement

The Andhra Pradesh State Irrigation Development Corporation is the operator of the system. State Government agencies and financial institutions have provided the funds required to develop the necessary infrastructure to operate this technology.

Effectiveness of the Technology

The effectiveness of this technology in achieving the objective of conserving water resources is evident from the fact that 28% of the total water requirement of the Krishna Delta was met through the conjunctive use of surface and ground waters, conserving 750 million m³ of surface water which could be diverted to other uses. In subsequent phases of this project, this savings is expected to reach 1 065 million m³.

Advantages

The advantages of this technology include the conservation of large quantities of good quality surface water in a very cost effective manner. This technology may also have application in places where the good quality surface water is not abundant.

Disadvantages

Application of this technology requires additional infrastructure, including a tubewell pumping system, which incurs additional costs. It also requires additional monitoring of the operation of the system to ensure the quality of the blended water, as some of the crops are sensitive to salinity and changes in water quality. Also, soil quality may be sensitive to the quality of the water applied, requiring that the ratio of surface to ground water volumes should be closely monitored and properly maintained to prevent salination of the soils.

Further Development of the Technology

In view of the large demands on the waters of the Krishna River upstream of Nagarjuna Sagar Reservoir, it has become necessary to utilize the groundwater supplies available in the Krishna Delta area to the extent possible. Based upon water quality considerations arising from the analyses of groundwater, canal water, and soil samples of the ayacut, it is possible to conjunctively utilize groundwater and surface waters in the Delta in blends of up to 28:72. Such use would conserve up to 1 016 million m³ of surface water for other purposes.

Information Sources

Vishwanadh, G.K. and D.V. Reddy 1995. Conjunctive Use of Ground and Surface Water in Krishna Delta for Irrigation, Journal of Indian Institution of Engineers, Civil Engineering Division, 7'5:197-202.