This document discusses 4 key ecohydrological principles for modeling water resources in drylands: 1. The response of dryland ecohydrology is slow due to long time scales and residence times of hydrological systems. 2. Dryland ecohydrological systems are nested hierarchical systems with different scaling properties than agronomy and meteorology. 3. Ecohydrological processes have strong feedbacks and non-linear responses, as actions can change the overall system. 4. It is important to balance the cycles of water and solutes to achieve sufficiency, sustainability, efficiency and resilience, as seen in the water and solute cycles of an example system in Windhoek.

1. Ecohydrological Principles in
Economic Models of Water
Resources in Drylands
C. Kuells
University of Freiburg, Chair Environmental Hydrology
WG 2 EU COST Action ES1104, Desert Restoration Hub

2. Scope and Objective
Simplify and cooperate
Simplify desert hydrology
for other disciplines
4 key principles
Conclusions and outlook
11.04.2013 2

3. 4 Key Principles
Ecohydrology of drylands
1. Time scales of system response
Response of dryland ecohydrology is slow
2. Scaling nested hierarchical systems
Different from agronomy, meteorology
3. Feedbacks and non-linear response
Action changes systems
4. Balancing cycles of water and solutes
Sufficiency, sustainability, efficiency, resilienc
e

4. 1.1 Hydrological Systems and Time
Residence time and response times are linked
CFC sampling
CFC water Henry’s law CFC air

5. 1.2 Groundwater Residence Times
10 to > 50 years for an active system up to 105 y

6. 1.3 Ecohydrological Response
Land use change and impact on recharge
Reduction of recharge Increase in recharge
1 (Smitt et al., 2002) 1 (Daves et al., 2001)
0.9 0.9
0.8 35 years 0.8
15 years
Degree of Impact
0.7 0.7 15 years 35 years
MRT
0.6 0.6
0.5 0.5
0.4 0.4
0.3 0.3
0.2 0.2
0.1 0.1
0 0
0 10 20 30 40 50 0 10 20 30 40 50
Years since impact
MRT=Mean Residence Time

7. 2.2 Topology of Ecohydrological Systems
Blue produces, orange consumes recharge

8. 2.3 Scale Effects
Complex scaling of unoff and recharge in drylands

9. 3.1 Feedbacks
Ecohydrological processes have strong feedbacks
Water and solute
balance of an alluvial
aquifer
Pumping (P) changes
transpiration (T),
recharge (T), storage (S),
lateral in- and outflow (L)
and vertikal seepage (V).
Benito et al., 2008: Comparing flood
recharge dynamics in two ephemeral
rivers in southern Africa: Implications for
Integrated Water Resource Management.
Water Research (subm.)

10. 3.2 Feedbacks in Ecohydro-Systems
Results of coupled modeling, Namaqualand
Less
salt
more
recharge
Thresholds
less plants
loss dy u=unexploited
e=evaporation
t=transpiration
d=depletion

11. 4. Ecohydrological System Cycles
Water and solutes in supply system of Windhoek
Lehmann (2011) Water Supply of Windhoek.

12. Conclusions
What does this mean for economic modeling?
• Check on time scales and residence times
• Scale and topology is important
• Systems change and system response changes
• Water and solute cycles need to be watched and
balanced (SSER objective functions)
These properties of ecohydrological systems in
drylands are not only constraints to observe …
They represent the key to ecohydrological system
management and desert restoration

13. Thank you for your attention
info@uhydro.de