Green Water Credits – exploring its potential to enhance ecosystem services by reducing soil erosion in the Upper Tana basin, Kenya

Figures & data

Figure 1. Location of the Tana River basin, reservoirs and land use.

Figure 2. Spatial expression of predicted erosion reduction, as compared with the baseline scenario, following ridging.

Figure 3. Spatial expression of the increase of recharge, as compared with the baseline scenario, following mulching.

Table 1. Modelled effects – percentages relative to the baseline – of different soil and water conservation scenarios on indicators related to green water, blue water and erosion. Values were calculated for relatively dry and wet years and cover different crops, as indicated (see text).

			Inflow Masinga	Sediment inflow Masinga	Crop transpiration	Soil evaporation	Groundwater recharge (basin)	Erosion (basin)
Scenario	Land use*	Year	MCM∙y^−1	Mt∙y^−1	mm∙y^−1	mm∙y^−1	mm∙y^−1	t∙ha^−1∙y^−1
Baseline		Dry	931	1.0	335	121	16	1.2
		Wet	2508	4.2	308	140	128	7.9
1 Bench terraces	MCT	Dry	1.1%	−21%	0%	−1%	3%	−23%
		Wet	1.9%	−21%	0%	0%	2%	−18%
2 Conservation tillage	M	Dry	0.1%	−1%	1%	−5%	1%	−2%
		Wet	0.1%	−1%	1%	−4%	0%	−1%
3 Contour tillage	M	Dry	1.1%	−10%	0%	−1%	7%	−12%
		Wet	0.8%	−7%	0%	0%	3%	−6%
4 Fanya Juu terraces and variations	MCT	Dry	0.4%	−21%	1%	−1%	4%	−23%
		Wet	1.3%	−20%	1%	0%	2%	−18%
5 Grass strips	MCT	Dry	0.6%	−11%	0%	−1%	3%	−14%
		Wet	0.6%	−10%	0%	0%	1%	−10%
6 Micro-catchments fruit trees	MCT	Dry	0.6%	−8%	0%	−1%	2%	−8%
		Wet	0.6%	−6%	0%	0%	1%	−5%
7 Mulching	MCT	Dry	0.4%	−6%	3%	−12%	3%	−9%
		Wet	0.5%	−6%	2%	−12%	2%	−8%
8 Rangelands	AR	Dry	0.1%	−4%	0%	−3%	1%	−4%
		Wet	0.0%	−2%	0%	−2%	0%	−6%
9 Ridging	M	Dry	1.4%	−18%	0%	−1%	23%	−21%
		Wet	1.0%	−12%	0%	−1%	10%	−12%
10 Riverine protection	MCTA	Dry	0.0%	−5%	0%	−1%	0%	−5%
		Wet	0.0%	−4%	0%	0%	0%	−4%
11 Trash lines	MCT	Dry	0.6%	−7%	0%	−3%	3%	−8%
		Wet	0.6%	−6%	1%	−2%	1%	−5%

Note: *M = Maize, C = Coffee, T = Tea, A = Agricultural arid and semi-arid, R = Rangelands.

Table 2. Benefit-cost analysis from the introduction of GWC scenarios based on the combined use of SWAT and WEAP analysis tools.

	Benefits – Costs (mUS$∙y^−1)
GWC scenario	Benefits	Costs	B-C
01_Bench	9.9	2.8	7.1
02_ConsTill	1.0	0.0	1.0
03_ContTill	4.9	0.0	4.9
04_FanyaJuu	9.0	3.7	5.3
05_GrassStrips	5.3	2.3	3.0
06_MicroCatch	1.6	0.1	1.5
07_Mulching	5.1	0.0	5.1
08_Rangelands	0.8	0.7	0.1
09_Ridging	8.9	1.6	7.3
10_Riverine	2.0	0.3	1.7
11_TrashLines	3.4	2.3	1.1

Figure 4. Total revenues (benefits minus costs) of implementation of the GWC scenarios for the four dominant water sectors (hydropower, domestic water supply and irrigated and rainfed agriculture).

Figure 5. Timeline illustrating developments after proposing and implementing a GWC approach in the Tana River basin, Kenya. K = knowledge community, G = governmental at different levels, E = enterprises and business, N = NGOs (for box numbers, see text).