Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas Universität zu Köln Lake Naivasha Hydro-Economic Basin Model (LANA-HEBAMO) - Status September Arnim Kuhn Institute for Food and Resource Economics Bonn University

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 2 Overview 1.Water management 2.The hydro-economic model 3.Applications and outlook

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 3 Hydro-economic modelling: Research tasks Consider conjunctive use of water resources within a numerical simulation model Develop long-term scenarios on the basin scale Quantify the resilience of the Lake Naivasha Basin‘s social- ecological system against droughts Simulate water management options Goals of water management: – Sustainability of farming activities in the Lake Naivasha Basin – Preservation of groundwater resources

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 4 A Hydro-Economic Basin Model -Structured as nonlinear optimization problem -Goal: maximization of incomes/utility from water and land use -Constraints involve yield formation, land availability and hydrological balances -‘Node network’ for spatial representation -Planning model with fixed market prices and costs

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 5 -Crops representative for the region (roses, vegetables, maize, peas, etc.) -Endogenous yield formation (water application per hectare, non-linear) -Simulation period: one year in monthly periods (fully dynamic) -Recursive-dynamic over a series of years -Carry-over of lake and groundwater fill levels between simulation years A Hydro-Economic Basin Model

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 6 Spatial structure: node network Elements (‘nodes’): lakes/reservoirs, river nodes (withdrawal), demand sites, aquifers Links between nodes determine possible water flows Irrigation water demand by the 12 WRUAs Water ‚input‘ by rain Water flows along rivers into the Lake Groundwater: uncertain

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 7 Households, industry Local groundwater aquifers Rivers, Lake Naivasha Farming areas Surface water use for irrigation Rainfall Evapotranspiration Infiltration of irrigation water Recharge of groundwater into rivers or the lake Infiltration of river and lake water Surface water for HH and industry Groundwater for HH and industry Groundwater use for irrigation Conjunctive use of water resources Inter-aquifer flows

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 8 Natural interaction between river and groundwater aquifers River flow direction ‚Bathtub‘ model of aquifers Shallow aquifers are mainly fed by the river! Inter-aquifer flows Recharge of GW by surface water Resurgence of groundwater

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 9

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 10 Optimization problem Use resources such that the sum of agricultural gross margins (GM) across farming communities r is maximized by taking into account constraints resulting from: agronomy (yield formation due to rainfall and water application) hydrology (hydrologic balances for lake, river nodes, aquifers, and fields) exogenous increase of non-agricultural water needs Encoded in GAMS, NLP-Problem, Solver Conopt3

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 11 Model development status Water sources: Rainfall (the ultimate source): historical rainfall data from different locations River water: fed by rainfall through runoff coefficients Lake and reservoir water: dimension of Lake and aquifers, Lake balance Groundwater (shallow and perhaps deep aquifers): data missing  Simplified hydrology: Evaporation rates dependent on locations within the basin: yes, but more local detail? Communication between lake/rivers and aquifers: not yet  Basin water balance: yes Spatial attribution (local conditions in WRUAs): weak 

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 12 Bio-economic features: Crop yields in outdoor irrigation cropping dependent on irrigation water use: first attempts made, crop parameters missing  Lake ecology dependent on the quality and extent of wastewater treatment: not yet  Fish catches dependent on the lake ecology: not yet  Model development status

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 13 My current headache... Goal: develop plausible scenarios for future supplementary irrigation demands Requires determination of crop water deficits under different rainfall and soil moisture conditions  agronomic question! Possible agronomic model: FAO AquaCrop Scientific partner in crop modelling: Crop science Bonn Results could also benefit ITC‘s basin model! Relation water  crop yields: Assumption: In the upper catchment, water demand for supplementary irrigation by smallholder will increase. This will increase the pressure on renewable water resource in the Basin.

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 14 AquaCrop: Example Maize

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 15 AquaCrop: Example Maize

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 16 How to use these results in LANA-HEBAMO?

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 17 Logistic approximations in EXCEL: Crop ET as a function of rainfall

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 18 Crop yield in % of max. yield as a function of crop ET Logistic approximation functions will be used in the GAMS code of LANA-HEBAMO

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 19 Issues in hydrology... Runoff coefficients as a function of rainfall (higher percentage runoff when rainfall is high) The old question: how to model groundwater? Correlation between groundwater and lake water levels (is it useful to bother about it)? Magnum mysterium: Seepage-out from the lake... really always 50 mio cbm per year?

Resilience, Collapse and Reorganisation in Social-Ecological Systems of African Savannas 20 Some aggregate simulation results Assumption: rainfall decreasing by 10% each year over 10 years 50% of Maize area has access to supplementary irrigation (not the case in reality now!) Increasing drought first increases demand for supplementary irrigation, but when water resources decrease ever more, irrgated areas will have to decrease Lake levels fall from 0.5m to almost 1m per year... even under average conditions!