Avoided runoff
Description​
The sponge effect in an ecosystem is the natural ability of the landscape to collect, retain, purify, and gradually release water. It acts as a buffer in water management: during heavy rainfall, the soil absorbs water to prevent flooding, and during dry periods, this stored water is gradually released.
Wet nature reserves, peatlands, forests, and healthy soils act like a sponge. They slow down the drainage of rainwater, preventing it from flowing directly into streams and rivers.
Some of that water will infiltrate further and replenish the groundwater. This is calculated in the infiltration service.
We do not calculate the water that can be stored on the surface due to flooding.
Qualitative valuation​
The qualitative score is calculated based on the potential runoff coefficient. Where complete runoff (runoff coefficient 1) is 0 and complete retention (runoff coefficient 0) is 10.
Quantitative valuation​
To calculate the avoided runoff, we use the method for calculating runoff in the Wetspass model. In this model, runoff coefficients (ROC) are calculated based on vegetation type, slope percentage, and USDA texture class. The amount of water that infiltrates is estimated as 1-ROC.
We then calculate the avoided runoff and the water that infiltrates as a weighted average of (1-ROC) based on the surface areas of different land use types multiplied by the average annual rainfall in the area.
Monetary valuation​
Currently, there is no monetary valuation because there are no estimates per cubic meter for flood damage.
Assumptions​
- We use the potential runoff coefficient. We do not take into account additional man-made drainage that occurs in an area.
- We translated the Belgian texture classes to the American USDA classes based on modeled mass fractions for sand, silt, and clay (in µm). We normalized the fractions so that the total sum = 100. Based on these fractions, a texture was assigned according to the USDA classification (Soil Science Division Staff, 2017).
Numbers to use​
Potential runoff coefficient (factor)
| Land use | slope% | sand | loamy sand | sandy loam | loam | silty loam | silt | sandy clay loam | clay loam | silty clay loam | sandy clay | silty clay | clay |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| forest | <0,5% | 0,03 | 0,07 | 0,1 | 0,13 | 0,17 | 0,2 | 0,23 | 0,27 | 0,3 | 0,33 | 0,37 | 0,4 |
| 0,5-<5% | 0,07 | 0,11 | 0,14 | 0,17 | 0,21 | 0,24 | 0,27 | 0,31 | 0,34 | 0,37 | 0,41 | 0,44 | |
| 5-10% | 0,13 | 0,17 | 0,3 | 0,23 | 0,27 | 0,3 | 0,33 | 0,37 | 0,4 | 0,43 | 0,47 | 0,5 | |
| >10% | 0,25 | 0,29 | 0,32 | 0,35 | 0,39 | 0,42 | 0,45 | 0,49 | 0,52 | 0,55 | 0,59 | 0,62 | |
| grassland | <0,5% | 0,13 | 0,17 | 0,2 | 0,2 | 0,27 | 0,3 | 0,33 | 0,37 | 0,4 | 0,43 | 0,47 | 0,5 |
| 0,5-<5% | 0,17 | 0,21 | 0,24 | 0,27 | 0,31 | 0,34 | 0,37 | 0,41 | 0,44 | 0,47 | 0,51 | 0,54 | |
| 5-10% | 0,23 | 0,27 | 0,3 | 0,33 | 0,37 | 0,4 | 0,43 | 0,47 | 0,5 | 0,53 | 0,57 | 0,6 | |
| >10% | 0,35 | 0,39 | 0,42 | 0,45 | 0,49 | 0,52 | 0,55 | 0,59 | 0,62 | 0,65 | 0,69 | 0,72 | |
| crop | <0,5% | 0,23 | 0,27 | 0,3 | 0,33 | 0,37 | 0,4 | 0,43 | 0,47 | 0,5 | 0,53 | 0,57 | 0,6 |
| 0,5-<5% | 0,27 | 0,31 | 0,34 | 0,37 | 0,41 | 0,44 | 0,47 | 0,51 | 0,54 | 0,57 | 0,61 | 0,64 | |
| 5-10% | 0,33 | 0,37 | 0,4 | 0,43 | 0,47 | 0,5 | 0,53 | 0,57 | 0,6 | 0,63 | 0,67 | 0,7 | |
| >10% | 0,45 | 0,49 | 0,52 | 0,55 | 0,59 | 0,62 | 0,65 | 0,69 | 0,72 | 0,75 | 0,79 | 0,82 | |
| bare soil | <0,5% | 0,33 | 0,37 | 0,4 | 0,43 | 0,47 | 0,5 | 0,53 | 0,57 | 0,6 | 0,63 | 0,67 | 0,7 |
| 0,5-<5% | 0,37 | 0,41 | 0,44 | 0,47 | 0,51 | 0,54 | 0,57 | 0,61 | 0,64 | 0,67 | 0,71 | 0,74 | |
| 5-10% | 0,43 | 0,47 | 0,5 | 0,53 | 0,57 | 0,6 | 0,63 | 0,67 | 0,7 | 0,73 | 0,77 | 0,8 | |
| >10% | 0,55 | 0,59 | 0,62 | 0,65 | 0,69 | 0,72 | 0,75 | 0,79 | 0,82 | 0,85 | 0,89 | 0,92 | |
| impervious | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |
| water | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
Translation to an indicator​
To give an idea of how much water is retained in the soil, we calculate the number of bathtubs that could be filled with that water. A bathtub holds 200 liters of water.
An example​
A 150-hectare pasture on sandy loam soil (texture L) is divided into 50 hectares of beech forest, flower-rich and species-rich grassland, and 50 hectares of marshland. The area has a slope of less than 5%. Sandy loam soil is classified as ‘sandy loam’ in the USDA classification. Suppose that the average annual rainfall is 880 mm/year.
The pasture retains 1,056,000 m³ of water in the soil. The new layout retains 1,100,000 m³ of water, which is an additional avoided runoff of 44,000 m³.
For the pasture: ROC= 0.2 1-ROC=0.8 water retained = (0.8 x 150 ha x10000) x (880 mm/year/1000)= 1,056,000 m³/year
For the future scenario ROC forest: 0.1 1-ROC=0.9 ROC grassland and moorland: 0.2 1-ROC= 0.8
Water retained in the soil = ((0.9 x 50 ha x 10,000) + (0.8 x 100 ha x 10,000)) x (880 mm/year/1000) = 1,100,000 m³/year
This is an avoided runoff equivalent to 220,000 bathtubs (44,000 x 1000/200)
More detailed models​
The current calculation determines the average amount of precipitation retained on an annual basis, based on annual average parameters. However, detailed hydrological models are available at VITO to calculate the water balance at a higher spatial and temporal resolution, taking into account the various components of the water balance. These models allow local hydrological processes such as runoff, infiltration, and groundwater flow to be taken into account, with the dynamic nature playing 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).