Recharge systems for protecting and enhancing groundwate

TOPIC 1
Alternative recharge systems / Recharge systems 205
METHODOLOGY
The technical effectiveness of the recharge structures is evaluated at two scales:
1. Impact at recharge structure scale;
2. Impact on the groundwater resources in the local aquifer.
Emphasis is thereby placed on the development of simple numerical solutions to help evaluate technical effectiveness,
which can be applied locally without detailed knowledge of artificial recharge processes.
Impact at recharge structure scale
At the recharge structure scale, the decline in water level in the structure is an indicator of the performance of a
recharge structure. The water balance for the structure can be simplified during periods of no precipitation and
when surface inflow and outflow can be neglected. A further simplification can be made if losses due to leakage,
direct abstraction etc. can be neglected, and if the structure is under effluent conditions in relation to the aquifer.
With these simplifications the water balance can be summarized as:
Infiltration = Change of volume of water in the structure - Evaporation
Under such conditions, the balance between evaporation and infiltration will determine the effectiveness of the artificial
recharge scheme.
The change of volume in the reservoirs at the Indian case study sites is monitored by recording the change in reservoir
water level with time. For periods without direct abstraction and rainfall, this is translated into infiltration rates
after subtracting estimated open water evaporation rates. Assuming the overall change in unsaturated zone storage
is negligible, this infiltration is equal to groundwater recharge.
Impact of artificial recharge on the groundwater resources
in the local aquifer
Besides estimating groundwater recharge from structures, the distribution of this additional water recharged is
important in determining its availability to the user. This was assessed by developing both analytical and numerical
models. Setting up of data intensive detailed numerical models to evaluate site-specific recharge schemes is not
always feasible, especially for low-technology schemes, e.g. small check dams, in developing countries. Emphasis
was therefore placed on the development of a simple numerical tool to allow the order-of-magnitude impact of artificial
recharge to be examined and visualized for aquifers of various hydraulic properties. The aim of developing an
analytical model was to provide a tool for assessing the impact of artificial recharge structures for users not familiar
with numerical modelling codes, such as MODFLOW (McDonald and Harbaugh, 1988). It allows users to investigate
homogeneous aquifer systems and their responses to recharge using Excel spreadsheets. The numerical model
was used to verify the analytical code by simulating responses of homogenous aquifers to recharge events and comparing
model results. Additionally, it was used to simulate the impact of artificial recharge on non-homogenous
aquifers and to incorporate abstraction from boreholes. The analytical and numerical models allow:
• Estimation of expected water level rise following recharge events within various hydrogeological settings.
• Assessment of the likely impact zone of a recharge structure, i.e. the area in which a rise in water level is
experienced.
• Simulation of the dissipation of the groundwater mound with time.
10 – 16 June 2005, Berlin ■ 5th International Symposium ■ AQUIFER RECHARGE ■ ISMAR 2005