MODELLING GROUNDWATER RECHARGE ESTIMATION USING MODIFIED SOIL MOISTURE BALANCE APPROACH IN OTUKPO BASIN

ABSTRACT

In this study,  groundwater recharge in Otukpo basin  has been estimated using a modified daily soil moisture balance based on a single soil water store for a climate classified as tropical with distinct dry and wet seasons in the Middle Belt part of Nigeria. Soil properties like field capacity, permanent wilting point, readily available water, actual and potential evapotranspiration, soil moisture deficit were all estimated and deployed in the model which algorithm was developed using Python programming language, hence the name modified soil moisture balance model. Runoff is estimated using runoff matrix and runoff coefficients which depend on rainfall intensity and soil moisture deficits. A new component, near surface storage, is used to represent continuing evapotranspiration on days following heavy rainfall even though the soil moisture deficit is high. Groundwater recharge is estimated for cassava and yam which are commonly cultivated vegetable crops in the study area. Meteorological data for the periods of 2008 to 2018 were used in the model analysis. The model recorded annual groundwater recharge which varied from 333.35 mm in 2009 water year (just 20.01% of annual rainfall for the year) to 38.119 mm in 2017 water year which is 3.6% of annual rainfall for the year). The highest annual rainfall depth was also observed in the year 2009 as 1665.4 mm, with the lowest annual rainfall depth, 1062.4 mm also observed in the year 2017. The annual runoff ranged from 322.04 mm in the year 2015, a 32.16 % of annual rainfall for the year to 935.56 mm in the year 2008 a 58.17 % of annual rainfall for the year. The lowest actual evapotranspiration AE was also observed in 2017 as against the highest in 2012. The AE ranged from 583.84 mm in 2017 to 721.39 mm in 2012. The model gave a simplified method of groundwater recharge estimation as well as runoff depth coupled with rainfall-runoff relationship.

CHAPTER ONE

1.0       INTRODUCTION

1.1       Background to the Study

Groundwater recharge is defined as water that infiltrates through the sub surface to the zone of saturation beneath the water table (Reese and Risser, 2010). It results in the increase of ground water storage and contributes to groundwater flow (Idowu, 2010). Groundwater recharge is a hydrologic process, where water moves downward from surface water to groundwater.

Groundwater is the primary source of water for domestic and agricultural water supplies throughout the tropics and much of sub-Saharan Africa (Doll et al., 2012) Efforts to meet projected increase in freshwater demand over the next few decades across sub- Saharan Africa depend on the development of the groundwater resource which in many environments is the only perennial source of freshwater. Groundwater is the capital source of freshwater for nearly half of earth’s population for irrigation and domestic water needs (Kunkel and Wendland, 2002). Groundwater is identified as a renewable

water resource for supporting agricultural, industrial, environmental and municipal domestic water demands

understanding the groundwater reservoir and forecasting it’s potential accessibility and

sustainability even though other elements have to be taken into accounts for example

,social, economic and hydrogeological considerations (Bogena, 2005).

Recharge is the primary method through which water enters an aquifer. This process usually occurs in the Vadoze zone, below plant roots and is often expressed as a flux to the water table surface.

According to Najjar (1999)   groundwater recharge also encompasses water moving away from the water table farther into the saturates zone. Recharge occurs both naturally through the water cycle and through anthropogenic processes in other words, artificial groundwater recharge where rain water and or reclaimed water is rooted to the subsurface.

Groundwater recharge happens when a part of precipitation on the ground surface infiltrates through the soil and the reaches the water table. Groundwater recharge can be known as water moving from the land surface to the unsaturated zone. When water reaches the water table, it can go out of the ground water to the surface water which is called discharge. The amount of recharge in humid region is usually high because the region receives large amount of rainfall, have favourable surface conditions for infiltration and a less susceptible to the influences of high temperatures and evapotranspiration (Reese and Risser, 2010). For example Azeez (1972) reported that a substantial rate of groundwater recharge occurs in the regolith overburden in the basement complex of Southwestern Nigeria. The estimation of the rate of natural groundwater recharge is a pre-requisite for efficient groundwater resource management (Kumar and Seethapathi, 2002). Hence, recharge estimate is peculiarly important in regions with large demands for groundwater supplies, where such resources are the key to economic development. While the estimation of recharge by whatever method is normally subject to large uncertainties and errors.

For many years, hydrologists have been trying to estimate natural recharge rates to aquifer system. In order to estimate the potential long term safe yield of an aquifer system however is related not so much to the proportion of the discharge that groundwater extraction centers are able to capture.

Groundwater recharge is an important component of the water balance and evaluation of groundwater resources largely depends on it (Dages et al., 2009). This estimation of the rate of natural ground water recharge is a basic pre-requisite for efficient groundwater resources management strategy that will ensure the protection of groundwater resources not only from climate change, but also from other stresses (Hennon, 2005).

Water movements in top soils determine the rate of recharge to the groundwater rate of plant transpiration, soil evaporation and run-off (Walker, 2002). Therefore an accurate description of unsaturated zone water movement and accurate methods for determination of parameter and input data are essentials to derive proper estimate of ground water recharge. Practicing hydrologists, typically make the best estimates of recharge possible by the use of methods that are relatively straight forward in their application and require only commonly available hydrologic data.   Arnold (2007) reported that groundwater recharge is the process by which water percolates down the soil and reaches the water table either by natural or artificial methods to replenish the aquifer with water table from the land surface. Most of the time groundwater is determined to a large extent on an imbalance at the land surface between precipitation and evaporative demand. In arid and semi-arid regions, the search for water which are under increasing stress from the growing human population poses a great challenge due to its scarcity (Corpo, 2010).

Groundwater as a dynamic system is located beneath the earth surface and moves under the control of many factors which are influenced by forces that are dependent on hydrogeology, hydrology and climatology. Recharge as one of the factors controlling the situation and fluctuates of groundwater is an important parameter that needs to be assessed more fully. Recharge occurring in small and large scales spatially and temporally is influenced by several factors such as meteorology soil characteristics, geology, surface cover, slope and depth of the groundwater level. Groundwater recharge estimation from precipitation is an integral part of hydrology and hydrogeology. Although precipitation is the most important source of groundwater recharge, the accuracy of currently attainable techniques for measuring recharge are not completely acceptable (Adeleke et al., 2015).

In measuring groundwater recharge, indirect methods are employed since there are no universally accepted standard methods. However methods for estimating groundwater recharges have been classified into three groups namely: physical model, obtained when the recharge is calculated from the base flow; chemical model method, which is used when the measurements of water soluble substances are considered; and numerical model methods such as HELP, RORA, PART, WEAP, WTF, PUISE and HYSEP, which can be used in the estimation of groundwater recharges for various climates such as in arid, semi-arid and tropical regions albeit each with its own advantages and disadvantages.

Groundwater recharge is a fundamental importance to meet the rapidly increasing agricultural, industrial and domestic water supply requirement within the Otukpo basin. This resource is almost the only key to economic development in the area and hence the estimation of groundwater is a necessity for the efficient and sustainable groundwater resource managements. Gehrels (2000) concluded that the method of estimating actual evapotranspiration and charges in soil water storage determines the accuracy of the water balance. However due to lack of basic understanding of the spatial and variability of hydrological processes, water management is becoming a major challenge. The groundwater recharge  estimation and causes of  groundwater level fluctuations in the Otukpo basin are not well understood due to limited knowledge of the soil water flow through the thick unsaturated zone and of the actual evapotranspiration from the area.

1.3      Aim and Objectives of the Study

The aim of this study is to estimate the groundwater recharge of Otukpo basin using the modified soil moisture balanced model alongside the hydrologic data.

The specific objectives of this study are to;

i.      Determine the relationships between hydrological parameters (Rainfall, Runoff, SMD, Recharge and ETo) in Otukpo Basin, and

ii.      Estimate  the  groundwater  recharge  within  Otukpo  Basin  using  modified hydrological model

The role of groundwater, with recharge estimation as a critical parameter for determining its sustainable use is becoming increasingly important in the emerging integrated water resource management. A proper understanding of estimating recharge as a result of modeling is crucial to assessing groundwater availability efficiently. The study would provide a better understanding of groundwater recharge estimation in the Otukpo basin and would also provide detail of how much groundwater that is available.

1.5      Scope of the Study

The scope of this work was limited to the estimation of the groundwater recharge within Otukpo basin using modelling written, using Python Programming Language.11 years Rainfall data of the study area was used in the modelling (2008-2018). Hargreaves equation was used for estimating evapotranspiration ETo.

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