Testing the Goodness of Fit of Infiltration Models for Soils Formed on Coastal Plain Sands in Akwa Ibom State, Southeastern Nigeria

Infiltration of water into the soil is an important physical process affecting the fate of water under field conditions, especially, the amount of subsurface recharge and surface runoff and hence, the hazard of soil erosion. The study was conducted to investigate the capability of six infiltration models, namely, Kostiakov, modified Kostiakov (A) and (B), Philip, modified Philip (A) and (B) to describe infiltration into soils formed on coastal plain sands parent material in Akwa Ibom State, Southeastern Nigeria. A total of 18 infiltration runs were made with the double infiltrometer technique. Model-predicted cumulative infiltration consistently deviated from field-measured data, that is, the models over-predicted cumulative infiltration by several orders of magnitude. However, there was a fairly good agreement between mean - measured cumulative infiltration (274.2 cm, CV = 35.5%) and Philip (405.6 cm, CV = 34.9%) and Kostiakov (480.3 cm, CV = 37.9%) models. The r2 values of the model parameters obtained from linear regression analysis were generally low. The data however, showed that the Kostiakov (0.49) and modified Philip ((B) = 0.48) and ((A) = 0.48) provided best fit with the field-measured data. The residual mean square error (RMSE) of the infiltration equations showed that the classical Philip model had the least non-significant value (6.47) while other models had significant (p≤0.01) values that range from moderately high (Kostiakov, 14.23) to very high (modified Philip (B) , 426.20). T-test of measured versus predicted cumulative intake showed all but the basic Philip infiltration model were significantly (p≤0.01) different from the fieldmeasured data, indicating the close agreement between the Philip model and the measured values. The results confirmed that Philip model could be used for routine characterization of the infiltration process on coastal plain sands parent material in Akwa Ibom State.