Knowledge journal / Edition 1 / 2022

A balanced approach to the groundwater balance

"The groundwater system is out of balance." "More is abstracted than recharged”. Or "we’re going to balance groundwater demand with groundwater supply." These are all statements that have recurred regularly in reports and press releases over the past year. There is still much misunderstanding about how a groundwater balance works. Using North Brabant as an example, we address three misunderstandings and show what role a groundwater balance can play when developing policies for water availability in areas of water scarcity.

Basic principles of groundwater balance in this article

• The groundwater balance describes the inflows and outflows of groundwater.
• It is conclusive by definition. Water cannot disappear.
• The balance is defined in space and time.
• It covers the fresh groundwater up to the impermeable geohydrological base.
• The balance varies. In winter, groundwater recharge takes place, causing groundwater levels to rise (and groundwater volume to increase); in summer, the opposite takes place. This averages out over several years, provided that external factors such as climate change or water use do not change in trend.
• Groundwater recharge is equal to the amount of precipitation minus evaporation and surface water drainage. In winter this is a positive number, in summer a negative number.

Misunderstanding 1: there is more abstraction than recharge

A good balance between abstraction and recharge is a logical precondition for a sustainable groundwater system and is a requirement of the EU Water Framework Directive (Article 4.1.b.ii). If more is abstracted than recharged, the groundwater system will eventually become exhausted ('mining'). Due to overuse, deep water tables and groundwater levels continue to fall, and the saltwater interface rises. This is happening on a large scale worldwide, including in California, northern India and around Beijing.

Figure 1. Groundwater balance (in millions of m3 per year) for North Brabant for an average year (top) and a further simplified groundwater balance to deep groundwater (bottom)

Figure 1 shows a highly simplified groundwater balance for North Brabant. Groundwater recharge (1) is by far the largest item in the water balance (1,686 million m³ per year). Groundwater abstraction for drinking water, industry and agriculture (6) mainly takes place from the thicker aquifers and amounts to 256 million m3 per year. Therefore, there is a large amount of groundwater being extracted, but the recharge is more than six times greater than the abstraction.
Theoretically, it is even possible to meet the entire need for groundwater in the Netherlands by abstracting water from the subsoil of North Brabant, taking into account a balance between recharge and abstraction. The result: extremely low groundwater heads, permanently dry streams, hardly any water in the Meuse, but still an equilibrium situation (Verhagen, et al., 2017). Abstracting more water leads to a new equilibrium with lower groundwater levels and thus less groundwater drainage to surface water. Simply put, the surplus of precipitation is divided between surface water drainage and groundwater abstraction. With more groundwater abstraction, the surface water drainage decreases.
A balance between abstraction and recharge does not always mean a sustainable situation. The consequences of the current groundwater abstractions are permanent low groundwater heads in deep aquifers, sometimes by as much as several meters. This has consequences for the entire water system. Areas that used to be fed by deep seepage have now become infiltration areas, groundwater levels have been lowered over a large area. This in turn affects the supply of groundwater to these areas, the amount of water drained by surface water and the amount of water available for nature and agriculture.

Misunderstanding 2: a balanced water balance is the key to sustainable water management

It is said: "There is more than enough precipitation in Brabant, but about 80 percent is drained, through ditches, canals and streams via the Meuse to the sea. The remaining water infiltrates into the soil and recharges the aquifers. But all this water is then withdrawn for agricultural irrigation, as drinking and industrial water and for watering gardens, sports fields and other urban green spaces."
Of the 1,686 million m³ a year of groundwater recharge, a net amount (drainage minus infiltration of surface water in water supply areas) of 1,488 million m3 per year is drained away via ditches, brooks and rivers (Figure 1). This leaves 198 million m³ a year for infiltration into the deeper groundwater. From that deeper groundwater, another 256 million m3 a year are extracted, which is 60 million m³ a year more than the deep groundwater recharge. This quantity is brought in horizontally from across the provincial border.
However, suppose the abstraction for drinking water, industry and agriculture is reduced by 60 million m3 a year. Groundwater levels will then rise and most of this volume will benefit from additional surface water drainage. A new situation arises, a new balance of the water system. Therefore, the recharge to the deep groundwater also decreases by about the same amount. In numbers: A reduction of 60 million m3 a year in abstraction also means almost 60 million m³ a year more drainage to surface water, partly from inflowing deep seepage water. In other words, there is no unique ratio whereby the recharge to deep groundwater, the abstraction of groundwater and the discharge by surface water are in balance.

Misunderstanding 3: surface water flow must be prevented

Stream flow is an essential part of the water cycle and is needed, for example, to provide streams with sufficient base flow (also a requirement of the Water Framework Directive).
Every drop of water is eventually drained or extracted, providing it does not evaporate. Surface water drainage can occur quickly or slowly; within a few days when a raindrop enters surface water via drains, or over many hundreds (to tens of thousands) of years when a raindrop eventually emerges as seepage via groundwater flow. Drainage therefore has many varieties; where does it come from and how long was its journey? The low-lying areas receive seepage water and naturally discharge more (surface) water than the core infiltration areas that lack surface water.
A general ratio between groundwater recharge and surface water discharge can therefore not be defined, because this is area-dependent. Managing the water balance is therefore primarily a distribution issue. A more robust water balance implies smaller differences between winter and summer seasons and longer times in the groundwater system (Stuurman et al., 2020). Ideally, sufficient water is stored in the spring, to provide stream flow in the summer.

Figure 2. The groundwater balance of North Brabant for summer and winter.

How to proceed?

The conclusion is that a general consideration of recharge and abstraction gives little indication for defining good groundwater status. There is no fixed ratio for abstraction and recharge. A further analysis per (catchment) area is needed to gain more insight into the requirements for groundwater levels, seepage and discharge. These requirements differ for each water and nature system. For example, upper stretches of streams are highly dependent on a sufficient and constant supply of groundwater, the base flow. Other areas require a sufficient seepage flow throughout the year with suitable groundwater quality.
An integrated assessment of groundwater and surface water is a requirement of the WFD. A systems approach including a water balance helps here. Areas of concern are the influence of groundwater management on the surface water system (the degree of drainage) and the influence of groundwater or groundwater-dependent nature areas (both quantity and quality). By defining the required availability of sufficient water of good quality in streams and nature, also in time, the water system can be better designed. Insight into the water balance and the size of the water balance items over time helps (Figure 2) and shows the following on the scale of North Brabant:
- Evaporation is a major issue in summer. Total evaporation exceeds the amount of precipitation in winter; small changes in groundwater recharge in urban and rural areas can significantly contribute to more groundwater recharge over large areas;
- The horizontal supply of groundwater from outside the province is small compared to the groundwater recharge within the province;
- Irrigation is a smaller item than abstraction for drinking water. The essential difference is that this water disappears from the water system through crop evaporation, while the drinking water eventually enters the Brabant surface water system as sewage effluent;
- The seasonal differences are great. Almost all evaporation and irrigation take place in the summer half-year. In the dry summer months, more groundwater is abstracted daily for irrigation than for drinking water production;
- The groundwater system is already being used as a large buffer for storing groundwater in the winter for reuse in the summer;
- The item drainage to surface water is large and aggregated to a single number. The timing and location of drainage vary and have a major impact on water availability.

Floris Verhagen
(Royal HaskoningDHV)
Roelof Stuurman
(Deltares)

Sources

- Floris Verhagen, Tom van Steijn, Joachim Hunink, Roelof Stuurman. Draagkracht grondwater Noord-Brabant. Royal HaskoningDHV rapport WATBF3125R003WM. 21 december 2017

- Roelof Stuurman, Floris Verhagen, Arjan van Wachtendonk, Han Runhaar. Een verkenning naar de Watervraag van de Noord-Brabantse Natuur. Deltares rapport 11203929-002-BGS-0002. 7 oktober 2020

Auteurs

Floris Verhagen
(Royal HaskoningDHV)

Roelof Stuurman
(Deltares)