Where, q is the infiltration rate (m/day), Q is the measured inflow to the pond (m3 day-1), A is the surface area of the pond (m2), H is the ponding depth (m), t is time (day), E is evapotranspiration (m day-1) and P is precipitation (m day-1). Evaporation from the ponds has been calculated as evapotranspiration using Hargreaves method (Hargreaves & Samani 1985). Linear regression analysis has been used to analyze the sensitivities of (1) Natural recharge, (2) Tidal effect of North Sea, (3) Pumping rate and (4) Pond water temperature on infiltration rate. Natural recharge varies seasonally as it is influenced primarily by precipitation and has been hence considered in the study. Tidal effect is suspected to have some impact on the groundwater levels since the site is very close to the North Sea. The shifting of saltwater – freshwater boundary may cause variation in daily groundwater levels in the area. Pumping rate is the main driving force in the movement of water in this system and has high potential to influence groundwater levels and infiltration rates. Temperature of water controls the fluidity of water and might have an impact on the hydraulic conductivity of the soil.
Natural recharge has been calculated by the SCS curve number method (Mockus, 2004). The area adjacent to the pond is a grassland with little vegetation and has dune soil. According to the hydrologic soil group (HSG) classification, the soil in this area represents group A which signifies lower runoff (Mockus, 2004). The curve number for this soil type and land use is 39 (NRCS, 1986). Calculation of natural recharge requires the potential of maximum retention and initial abstraction to obtain the contribution of rainfall to runoff and infiltration.
Tidal effects are evaluated by using daily sea level information obtained from sea level station monitoring facility at Oostende, which is 21 km away from the St-André MAR facility, along the coast of North Sea. Pumping rates and water temperature have been measured on site by the IWVA (Intercommunal Water Company of Veurne-Ambacht), who manages this site. Viscosity and density are two factors that are directly influenced by temperature. Dynamic viscosity of water is calculated using the International Association for the Properties of Water and Steam 1997 (IAPWS 97) and density of water is calculated using the equation given by Maidment (1993).
The aforementioned parameters were analyzed using single variable linear regression to develop a relationship between two variables by fitting a linear equation individually to each observed parameter. Coefficient of determination (R2) has been used as a statistical measure to judge the sensitivity of infiltration rates to the parameters. A second method of multivariate regression was used to verify the findings, where the sensitivities of parameters are assessed in combination. For this method, p- value has been used as the statistical measure, such that the variable with the lowest p-value is the most sensitive. A p-value of 0.05 indicates a 95% probability of the variable to have some effect on the parameter it is being compared to (Abbaspour, 2007).