Effect of slope on infiltration, models and hydraulic properties on coastal plain sands in Akwa Ibom State.

INTRODUCTION

Water is one of the principal factors limiting the growth of plants not only in arid and semi arid region, but also in the humid environment where poor moisture distribution and low moisture availability to plants bring about water stress. Therefore there is need for good management practices in order to solve-water-related problems such as irrigation planning and erosion control.

            Slope is an important component of soil management and conservation because its influences infiltration, runoff, and soil drainage, irrigation and erosion, use of machinery and choice of crops. Slope relates to the configuration of the land surface and is described in terms of differences in elevation, topography and landscape position, in other words the lay of the land (Brady N.C, 1990). It can also be described as the incline or gradient of a surface and is commonly express in percent. Slope may take different shape i.e. convex, concave or plain shape and may be described as simple slope (slope with smooth appearance with surface extending in one or two direction) or complex slope (slope which extend in several directions). Whichever form it manifests it may either hasten or retard the infiltration characteristics of the area. For example steep slope generally encourages rapid soil loss by erosion and allows less rainfall or irrigation to enter the soil before running off the surface this result in poor infiltration and poor ground water storage (Nyle et al., 2003). Sloping lands generally experience problems of poor infiltration, high runoff, and soil erosion as a results of the disintegration of soil organic matter that aids in soil aggregation and water retention thereby encouraging water to run over the soil surface and erode important soil properties that bind the soil together for better infiltration. This in turn affects crop water status, performance and yield. Slope also influences exchangeable potassium (K), organic carbon (C) and clay content that aid in soil aggregation (Boiling, et. al, 2014).

            Soil hydraulic properties are critical components of mathematical models for studying or predicting water flow and solute transport processes in the subsurface zone. One of the most commonly measured soil hydraulic properties is infiltration, among others such as saturated and unsaturated hydraulic conductivity (Bormann et. al., 2008).

            Water measurement in the soil through infiltration is an important indicator for efficient irrigation and drainage systems. Understanding and modeling of hydrological processes in fields are crucial in studying surface run-off normally difficult to measure accurately due to variation of infiltrated-related soil physical properties from site to site in the field (Igbadun and Idris 2007; Leke et al., 2003; Arab et al., 2014). Infiltration is the process of water movement from the ground surface into the soil, while percolation is the process of water flow from one point to another within the soil (Haghaibi et al., 2011; Ogbe et al., 2011; Adindu et al., 2013). Variations in infiltration are cause by precipitation, land use and vegetation type (Leke et al., 2013). Cumulative infiltration is the total quantity of water that enters the soil in a given time, thus, infiltration rate and cumulative infiltration are two parameters commonly used in evaluating the infiltration characteristics of soils (Ahameku, 2011).

            Infiltration characteristics of soils are quantified when field infiltration data are fitted mathematically to infiltration models (Ogbe et al., 2011). There exist ample studies for the evaluation of infiltration models either for the purpose of establishment of model parameters. Validating models or comparing models efficiencies and applicability for different soil conditions (Mudiare and Adewumi, 2000; Usman et al., 2011; Stanley, 2015). For instance, Igbadun and Idris (2007) investigated the capacity of Kostiakov’s, Philips, Kostiakov-Lewis’ and modified Kostivakov infiltration models to describe water infiltration into a hydromorphic soil of the flood plain in Zango, Zaria, Nigeria, Oku and Aiyelari (2011) deduced that Philip’s model was more suitable than Kostiakov’s model from a study of the infiltration rate of the soil of the humid forest in southern Nigeria.   

            Musa and Adeoye (2010) in their study to adapt infiltration equations to the soil of permanent site farm of the Federal University of Technology, Minna, in the Guinea Savannah zone of Nigeria, stated that Kostiakov model showed a better performance over the Philip and Horston models. Mbagwu (1997) reported that Philip model would always fail to predict measured infiltration when the assumptions of the model are not met during the infiltration process.

            Estimation of crop water requirement in dry lands need information about soil infiltration characteristics, which are useful in the management of newly irrigated and during their growing (Leke et al., 2013). The need for in-depth and field-specific study of infiltration models is very essential for coastal plan sands of Akwa Ibom State as model parameters and performance vary for different soils and time (Ogbe, 2011). However it has been observed that the increasing soil erosion and soil degradation menance experience in Akwa-Ibom State whose coastal plain sand covered over 75% of the total land mass is due to the inability of the soils to accept infiltration as a results of slope and certain soil characteristics that discourages water intake rate by the soil making the water to flow over the land surface during rainfall or irrigation activities. Therefore ascertaining the effect of slope on infiltration characteristics, models and hydraulic properties is critical in the management and conservation of Akwa-Ibom State coastal plain sand soils; as it will aids in making informed decision toward erosion controlled, improving crop yields, and suggesting irrigation design that will be suitable in the area.

            Direct infiltration measurement is laborious, tiresome, time consuming and could be expensive particularly where water is a limiting factor. Several methods for measuring infiltration have been developed which include: Rainfall simulator, run-off plots, basin method, furrow method and cylinder infiltrometers (Arab et al., 2014). The choice of any method depend on the purpose of study. The double ring cylinder infiltrometer in a homogenous soil within a measured area was adopted in this study because it is cheap to construct, easy to operate and transported (Li et al., 2010).