5. Water Fluxes

This section describes the methods to estimate fractions of water fluxes that end up in surface runoff, or leached into soil. The approach was originally described by Velthof et al. (2009), and modified in Review and further differentiation of pedo-climatic zones in Europe (2011, pp 63–66).

Surface runoff

Surface runoff occurs when rainfall exceeds the maximum infiltration level of the soil. In the Miterra models, a maximum runoff factor is determined based on slope classification, and then the actual runoff fraction (Lrunoff) is estimated by applying a group of reduction factors.

Equation 5.1

where:

is the maximum runoff factor dependenent on slope.

Slope Percentage (%) [1]

[0, 8)

[8, 15)

[15, 25)

≥ 25

Slope Class

1

2

3

4

Lrunoff(max)

0.10

0.20

0.35

0.50

[1] Slope Percentage = tan(Slope Degree) × 100.

is a reduction factor for land use or crop.

Land use

Grassland

Other land use

flanduse

0.25

1.0

is a reduction factor for precipitation surplus.

Precipitation Surplus (mm)

[0, 50)

[50, 100)

[100, 300)

≥ 300

fprecip

0.25

0.50

0.75

1.0

is a reduction factor for soil type/texture, which is based on the clay content of the soil.

Soil Texture

Peat Soil

Clay Content (%)

< 18

[18, 34)

[34, 60)

≥ 60

ftexture

0.25

0.25

0.75

0.9

1.0

is a reduction factor for the depth to rock.

Depth to rock (cm)

< 40 [1]

≥ 40

frock

1.0

0.8

[1] The threshold value of 25 cm should be used, but ESDB only reports this value in 40 cm intervals.

For a heterogeneous region with n distinct subareas (e.g., a NUTS region with more than one land use type or soil texture class), reduction factor for the region is calculated as an area-weighed average of all subareas:

Equation 5.2

where:

is the reduction factor for the i-th subarea.

is the fraction of the i-th subarea to the total (agricultural) area of the region.

Leaching

Similar to surface runoff, leaching fraction is estimated by applying a group of reduction factors to a theoretical maximum leaching.

Equation 5.3

where:

is the maximum leaching factor per soil texture type, which are based on ESDB “Dominant surface textural class” (database field TEXT-SRF-DOM).

ESDB Texture Class Miterra Texture Class Lleaching(max)

(1) Coarse

Sand

1.0

(2) Medium

Loam

0.75

(3) Medium fine

Loam

0.75

(4) Fine

Clay

0.5

(5) Very fine

Heavy clay

0.5

(9) Peat soils

Peat

0.2

is a reduction factor for land use. It has a fixed value of 0.36 for grassland, and 1.0 for other land use.

is a reduction factor for precipitation surplus, which differs per soil texture class (see Miterra texture class above).

Precipitation Surplus (mm)

[0, 50)

[50, 100)

[100, 300)

≥ 300

fprecip

Sand & Loam

0.25

0.50

0.75

1.0

Peat, Clay & Heavy Clay

0.25

0.75

1.0

0.5

is a reduction factor for average annual temperature.

Average Annual Temperature (°C)

< 5

[5, 15)

≥ 15

ftemp

1.0

0.75

0.5

is a reduction factor for maximum rooting depth. Rooting depth data are based on ESDB “Depth class of an obstacle to roots” (database field ROO).

ESDB Depth Class Rooting Depth Class froot

(4) Obstacle to roots between 20–40 cm depth

Shallow

1.0

(3) Obstacle to roots between 40–60 cm depth

Moderate

1.0

(2) Obstacle to roots between 60–80 cm depth

Deep

0.75

(1) No obstacle to roots between 0–80 cm

Very Deep

0.75

is a reduction factor for soil organic carbon content. SOC data are based on ESDB “Topsoil organic carbon content” (database field OC_TOP).

ESDB SOC Class fsoc

(H) High (> 6%)

0.5

(M) Medium (2–6%)

0.75

(L) Low (1-2%)

0.9

(V) Very low (< 1%)

1.0
For a heterogeneous region, reduction factors are calculated as area-weighed averages ( Equation 5.2).