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).