Do Solar Powered Irrigation Systems (SPIS) Contribute to the Overexploitation of Groundwater Reserves

Introduction

The common argument against the utilisation of SPIS is that this technology goes hand in hand with overexploitation and eventually depletion of limited water resources such as groundwater. Concerns are based on the following reasoning: SPIS lead to free pumping and hence an overexploitation of groundwater reserves is inevitable or at least very likely.

From a resource economist perspective, this argument is known as the Herder Problem or “the Tragedy of the Commons”  first described by Hardin in 1968: If there is a finite common-pool resource (e.g. groundwater) where it is difficult and costly to exclude potential users, this resource will eventually be exhausted because the rational individual will maximise his/her own utility rather than conserving the resource for the benefit of all. Meaning that an open-access resource will inevitably be over-exploited unless access and use of this resource are restricted by some form of governmental regulation or the allocation of property rights .

As an effect of the over-exploitation of a groundwater resource not only the environment but also the economy and society within the affected region have to suffer from several effects:

• Unsecure water availability through drying wells and springs increase the risk of crop failure;
• Aquifer salinization and seawater intrusion with long-term implications for agricultural productivity;
• Increased risk of conflicts between different users (e.g. farmers, domestic water supplier, industrial users);
• Environmental impacts on groundwater-dependent ecosystems, such as drying up of wetlands and river base flows .

The following argumentation will further show that all kinds of irrigation – and not just the solar powered one – face the same problems. The general principle is that the smaller the restrictions (lack of a mechanism for control of consumption or fee for use) of a water provision system, the greater the consumption will be . For that reason all irrigation in general requires the integration of principles of sustainable water management. Especially if groundwater regulation and protection in target countries is weak or even absent .

Definition of Groundwater

First, there are some general problems concerning knowledge on groundwater in principle.

Groundwater is usually defined as the water contained in underground reservoirs, whereas surface water bodies are rivers, lakes or reservoirs. The hydrological cycle links surface and groundwater and its dynamic nature makes categorical definitions problematic. Through seepage into the ground surface water can recharge aquifers e.g. when water seeps in the river bed or flood water percolates. The other way round, groundwater can become surface water through discharge into rivers, lakes and springs. So the flow of both systems overlap and they are not fully additive .

Besides the issues of precisely defining what groundwater is, there are major uncertainties on the volumes and spatial distribution of both groundwater recharge and withdrawals . Accurate data is missing and as Morris er al. argue, this chronic lack of data on groundwater conditions and trends results from an undervaluation of groundwater and the time lag between cause of the problem (e.g. over-abstraction) and effects (e.g. falling water levels).

Challenges of Groundwater Pumping

Arguments against the utilisation of SPIS concerning groundwater over-exploitation are actually arguments related to all water supply systems that run on groundwater in an ecologically unsustainable way. There are several issues that need to be considered when installing a water supply system of any kind (domestic and/or agricultural use) in order to operate in a way that the utilised water resource is not over-exploited. For the purpose of this work, the following arguments are related to the installation of groundwater-fed irrigation systems but could be generalised for other water resources and uses.

The evaluation of the water resource to be used should be the first step of any water supply project which should be done even before the conception of this project starts. The question here is e.g., if the water quality is appropriate for its planned usage. Possibly due to the already above mentioned lack of data, many studies fail to acknowledge and evaluate groundwater resources over the length of a project both in terms of quality and quantity which in consequence leads to the failure of the project .

The second reason why irrigation systems could be sized wrongly is that the water resource’s parameters are not taken into account. These parameters are the static and dynamic level of the water resource and its aquifer recharge (replacement) capacity. They depend on the composition of the geological substrata, rainfall patterns and topography and can undergo great variations. Because these parameters can be crucial for a good configuration of the system, capacity testing of the well prior to the configuration of the equipment is needed . In order to estimate the recharge capacity, the responsible authorities need to have as-accurate-as-possible data on the following:

• The total groundwater extraction by humans (via pumping) and springs,
• high resolution data on precipitation, evapotranspiration and runoff (land-use and vegetation/crop mapping of the groundwater basin helps to quantify these variables more exactly),
• hydrogeological data on the groundwater flow and underground storage characteristics (e.g. derived from pumping tests) .

Groundwater-fed irrigation does not only directly reduce the availability of groundwater, it also affects the local hydrological cycle. The surface soil moisture in irrigated areas is increased and thus evapotranspiration which increases the flux of the local hydrological cycle. Even though through seepage a part of the irrigation water flows back to the aquifer, groundwater pumping produces short-term decline in local groundwater levels. From the perspective of water balance, increases in evapotranspiration means that groundwater abstraction is greater than the replenishment and in the long run this can cause falling water tables. Groundwater pumping also affects the permafrost which causes a wide range of ecological problems .

The next detail that could lead to an over-exploitation of the groundwater resource is the lack of understanding of specific crop water requirements. If the amount of irrigation water needed is estimated incorrectly, the irrigation system is configured wrongly and the volume of groundwater abstraction might possibly be too great .

Another very important aspect which has to be considered is that in rural contexts water supply systems for production and domestic consumption are usually combined . This results from the fact that because of natural purification processes much groundwater is of good quality what makes it a valuable source of potable water . Thus the level of demand has to be analysed with regards to that link.