Infiltration and stable water levels

Description

Infiltration of rainwater into the soil is an important ecosystem function. Infiltration ensures sufficient ground and surface water. After all, the water finds its way to deeper groundwater layers and thus ensures that sufficient drinking water is available. Groundwater partly returns to the surface in seepage zones and thus contributes to a stable water level in the watercourses. The stabilization of both ground and surface water levels is a support service for many other services: water supply, shipping, avoided damage from drought, protection against salt intrusion, etc.

We value this service by estimating the amount of water that infiltrates locally into the ecosystem annually (m³/ha.year).

Qualitative valuation

We base this on the method developed in ECOPLAN. The extent to which water can infiltrate into the soil partly depends on the physical system and partly on the land cover and use. We calculate the potential infiltration based on the physical factors. Then we will look at the effects of the land cover.

The most important physical effects are:

• Soil texture: e.g. in a sandy soil, water will infiltrate much faster than in a clay soil. For Flanders, the average annual groundwater recharge was determined for various soil texture classes in Batelaan, Meyus et al. 2007.
• The depth of the groundwater table: the presence of shallow groundwater limits infiltration. The minimum of these two effects is considered as potential infiltration.

To calculate the current infiltration, we look at the amount of rainwater that can effectively reach the soil due to the land use. Here we take into account the effects of water interception. If the water that can reach the soil is smaller than the potential infiltration, this will be limited. If not, the actual infiltration is equal to the potential infiltration.

Monetary valuation

If there is not sufficient groundwater available and/or the flow rate in our waterways is low, water users will have to get their water from elsewhere. To value this ecosystem service, we use the price that a drinking water company must pay to purchase water elsewhere, which is €0.55/m³.

Assumptions

• Unlike the ECOPLAN method, we do not take into account paved surfaces that would drain to a sewer. We assume that in a rural context these drain into the surrounding land.
• We start from an average precipitation amount of 450 mm (average precipitation amount - evaporation before reaching the bottom)

Numbers to use

Table: Qualitative valuation score

infiltration in m³/ha	score
0	1
500	2
1000	3
1500	4
2000	5
2500	6
3000	7
3500	8
4000	9
4250	10

Table: Qualitative valuation score

soil texture	max. infiltration m³/ha
You	750
E	1880
A	2250
L	2630
G	3000
P	3000
S	3380
Z	3750
X	4500
V	1500
W	1500
OB	0
OT	0
OE	0
ON	0

ECOPLAN calculation based on Batelaan, Meyus et al. 2007

Formula to use to calculate the maximum infiltration due to groundwater levels: If GLG>100: GHG (in cm)x4 +100 Otherwise: GHG x4 + GLG Multiply the obtained value by 10 to convert to m³/ha. The value can be a maximum of 4500 m³/ha. This is automatically the case if the GHG > 100

Table: Inception of land use

Land use	Inception (mm)
Grasslands and tall herbs	100
Birch	200
Elm	200
Common ash	200
Beech	150
Oak	200
Poplar	200
Alluvial forests (alder, willow, ...)	200
Other deciduous/mixed deciduous trees	200
Douglas fir	250
Norway spruce	300
Scots pine	250
European larch	225
Corsican pine	250
Austrian pine	250
Silver fir	250
Other coniferous/mixed coniferous	250
Mixed forest	225
Heathland	50
Shrubs (including bird cherry, hawthorn, gorse, gale, sea buckthorn, ...)	175
Reed	250
Other wetland vegetation	100
Flat land and marshes	100
Lakes	50
Rivers	50
Flax and hemp	100
Potatoes	100
Sugar beets	100
Ornamental plants	100
Zucchini/pumpkins	100
Herbs	100
Vegetables lowN	100
Vegetables group 1	100
Vegetables group 2	100
Vegetables group 3	100
Other vegetables and herbs	100
Grains, seeds and legumes	100
Maize	100
Fruit and Nuts	125
Fodder	100
Silo corn	100
Green cover	100
Other crops	100
High density orchard	125
Traditional orchard	200
Community gardens	100
Wasteland or agricultural road	25
Meadow/temporary grassland	100
Woodside, wood wall and other high green coniferous	250
Woodside, wood wall and other high green deciduous trees	200
Pools, ponds and canals	50
Verges and other elements of grasslands and tall herbs	100
Other low green	100
Row of trees	200
Tree row larch	250
Tree row spruce	250
Tree row scots pine	250
Hedgerows and bushes	175
Wall vegetation	100
Sparsely vegetated land (incl. beach, dunes, etc.)	25
Buildings	9*
Green houses	9*
Gardens residential	175
Gardens other	175
Roads and squares	9*
Other urbanized area	9*
Other high green	200

*Paved area obviously does not retain water, but the water cannot infiltrate either.

Translation to an indicator

To get an idea of how much water infiltrates, we compare it with the annual tap water consumption of an average family (2.3 people) in Flanders. This is 73 m³/year (VMM Water Book 2020)

An example

For the example, we refer to the Dutch version of the manual.

More detailed models

The water that infiltrates into the soil and into the groundwater is determined on the basis of the total water balance. In the current calculation, the average amount of precipitation that infiltrates is calculated on an annual basis, based on annual average key figures for soil texture, land use and average groundwater level. However, detailed hydrological models are available from VITO to calculate the water balance at a higher spatial and temporal resolution, taking into account the different components of the water balance. These models allow to take into account local hydrological processes such as runoff, infiltration and groundwater flow, where the dynamic character plays an important role, resulting in more accurate calculations and scenario analyses. Setting up such hydrological models requires a certain amount of time and budget. For more information, contact VITO (jef.dams@vito.be).