ABSTRACT
Soil water characteristics are critical hydraulic properties governing soil water availability and movement in soils. Sustainable soil water conservation would not be possible without accurate knowledge of these hydraulic properties. Study of soil properties such as field capacity (FC), permanent wilting point (PWP) and hydraulic conductivity (Ks) play important roles in soil moisture retention. These parameters can be measured directly; their measurement is difficult and expensive. Thus, Saxton and RawlSoil Water Characteristics-Hydraulic Properties Calculator (SWC-HPC) model provided another alternative for estimating soil parameters from more readily available soil data. In this study, 180 soil samples were collected from three different sites locating at Minna (Ferruginous tropical soil), Badeggi (Hydromorphic soil) and Mokwa (Ferrosol) which represent the major soil types in Niger State. Laboratory values of soil water characteristics were obtained from thenineprofile pits dug at (2m x 1m x 1.5m).One bulk sample and three undisturbed soil core samples were taken at a depth of 20cm progressively down the profile to a depth of 100cm. Soil samples were analysed for percentage sand, clay, and silt, as well as organic matter, salinity and compaction which were the independent variables in the model while wilting point, field capacity, saturation point, hydraulic conductivity, bulk density and plant available moisture content were the dependent variables.The model out puts were statistically compared with observed parameters from laboratory tests using the root mean square error (RMSE), coefficient of variation (CV), modelling efficiency (EF), coefficient of residual mass (CRM) and chi-square. The model accurately predicted the observed bulk densities of the soil tested, satisfactorily simulated soil moisture content at permanent wilting point and moderately simulated plant available water. The model however poorly predicted the saturated hydraulic conductivity and soil moisture content at field capacity of the soil tested. The SWC-HPC may therefore be used only to simulate soil bulk densities; moisture status at permanent wilting point and plant available water values in the three study locations of Niger State.
CHAPTER ONE
1.0 INTRODUCTION
1.1 Background of the study
Nigeria, a country located between latitudes 40 to 140N and longitude 20 to 140 E with a total area of 923,768 km2 (Food And Agriculture Organization., 2006) contains six ecological zones, ranging from mangrove swamps belt and tropical forests along the coast to open woodland and savannah on the low plateau which extends through much of the central parts of the country to semi arid plains in the north and high lands to the east (United States Department of Agriculture, 2006). These ecological zones can be divided further into various sub-zones according to FAO findings. The major sub-zones in Nigeria are fresh water swamps, forest, lowland rainforest, mangrove forest, motene savannah, Guinea savannah, Jos Plateau, Derived savannah and Sahel savannah (FAO, 2006). The savannah zone of Nigeria cuts across the west, east and northern part of the country and covers an area of about 700,000 km2(FAO, 2006).
The whole of the savannah covers about three-quarters of Nigeria’s total land area. The soils are characterized by low activity clay with kaolinite and sesquioxides (FeOH) forming 80-90% of clay fraction (Moberge and Esu; 1991) and highly weathered with soil types ranging from loamy sand to sandy loamy in the top. The greater amount of kaolinite and sesquioxides leads to lower cation exchange capacity (CEC) of the soils (Enwezor, Udo, Ayotade, Adepelu, and Chude; 1990). Soils in the moist guinea savannah are characterized by coarse textured surface soil with low activity kaolinitic clay; high base status, low effective cation-exchange capacity, deficiencies of N,P,K,S and Zn which are high sub- capacity to soil compaction and erosion, because the soil does not have stable structures (Tian, Kang, Akjobundu and mimanyoung, 2005). The surface soil in the Guinea savannah is generally sandy, this accounts for high runoff at the top soil with low water holding capacity hence soil water can be lost to evaporation (Salako, Ghuman, and Lal, 1995).
Soils in the moist Guinea savannah have their origin from igneous and sedimentary rocks, these rocks account for the presence of recent alluvium, Nupe Sandstone and Basement Complex (Reconnaissance Soil Survey of Nigeria (1990) Soils like Alfisols, Ultisols, Oxisols, Entisols, Inceptisols and Vertisols are soil types usually found in the moist Guinea savannah zone, (Salako et al., 1995, Tian et al., 2005; FAO 2006). These soils are usually seen at the surface as dark brown or dark yellowish brown, sandy loam and moderate fine texture. (Reconnaissance Soil Survey of Nigeria, 1990).
In Nigeria, bush fire, deforestation, increasing intensity of cultivation (Senjobi, Adeokun, Dada and Ogunkunle, 2007), tillage related practices (Lal, 1986; Khurshid, Iqbal, Arif and Nawaz, 2006), low input agriculture, accelerated erosion (Fahnestock, Lal and Hall, 1995) and construction work are summarized as causes of land degradation.
Based on the above, for agricultural activities in the zone to increase in terms of production of crops, irrigation practices and use of machinery, the soil water characteristics must be studied alongside other soil properties, hence the need for evaluation and model validation.
Soil-water characteristics refer to the relationship between soil and the moisture content present in it. This relationship is commonly represented with curve, referred to as soil-water characteristics curve (SWCC); and it shows the moisture contents of the soil at different suctions. The moisture content defines the amount of water contained within the soil pores. In soil science, the term volumetric water content, O, is commonly used for moisture content, while in geotechnical engineering practice, the term gravimetric water content, w, which is the ratio of the mass of water to the mass of solid, is used. Another terms commonly used to indicate the percentage of void that are filled with water is degree of saturation, S. the suction, also referred to as soil water potential, is related to the pressure that will be exerted to remove mixture from the soil. It may be express either as matric suction (also known as capillary pressure) or total suction (matric suction plus osmotic suction). Soil suction may range from zero kilo-Pascal (kPa) for moisture content as saturation (when all soil pores are filled with water) to about 1,000,000 kPa at zero moisture content (all soil pores are filled with air) (Fredlund, and Xing, 1994).
In Agriculture, the soil moisture content between two points on the SWCC is of great importance to crop growth and development. These points are air entry point and residual moisture content point. The air entry point on the SWCC (i.e. the bubbling pressure point) is the matric suction where air start to enter the large pores in the soil (Fredlund and Xing, 1994). The moisture content at this point is commonly referred to as the field capacity (FC). The residual moisture content point on the SWCC is the moisture content where a lager suction (=1,500 kPa) is required to remove water from the soil. The moisture content at this point is known as permanent wilting point (PWP). The moisture content of the soil between the FC and PWP is referred to as available water (AW). This is the water available for plant to take up for its metabolic activities. Information on the AW of soils are required in planning irrigation scheduling for crops, design of irrigation systems, drainage systems and other soil and water management strategies(Igbadun, Oyebode, and Mohammed, 2011).
Soil moisture is fundamental in several disciplines of the environmental sciences. Unsaturated soil behaviour such as shear strengths volume change, diffusivity and adsorption are related to the soil water characteristics (Fredhund and Rahardjo, 1993). Soil water characteristics contain important information regarding the amount of water contained in the pores at a given soil suction and the pore size distribution corresponding to the stress state in the soil (Fredlund, Wilson, and Fredlund, 2002).
Accurate estimates of soil moisture content are necessary for meteorological, hydrological, climatological, ecological and agricultural research and operations. The largest spatial and temporal variability of soil moisture caused by heterogeneity of soil texture, topography, vegetation and climate in the natural environment makes it difficult to measure and so dynamic models have been used as an alternative in many applications. However, the results obtained by dynamic modelling depend heavily on the quality of the input data used (Abbott and Refsgaard, 1996).
The accuracy of soil water estimation is important in order to improve weather, climate and hydrological models. For example, Rown-tree and Bolton (1983) showed that small error in the soil moisture initialization could lead to large error in weather forecasts. Also, Moberg and Jones (2004) demonstrated that unrealistically high day time maximum temperature were simulated as a result of excessive drying out of soil.
In agriculture, knowledge of soil moisture patterns allows more efficient planning of irrigation scheduling and better crop yield forecasting. Knowledge of spatial and temporal evolution of soil moisture would be beneficial for precision agriculture which is based on the concept of soil specific management within a field according to specific site conditions in order to maximize population and minimize environmental damage (Vrindts, Reynier, Darius, Baendemaeker, Gilot, Sadaoni, Frankinet, Hanquet and Destain, 2003.). For mechanized farming, it is important to know the strength of the soil which depends partly on its moisture content. Soil moisture conditions could serve as a warning for subsequent flooding or drought if the soil has become too saturated or too dry (Richter and Semenov, 2005).
1.2 Statement of Problem.
Over the years, soil-water characteristics are determined through laboratory procedures carried out on collected soil samples. These procedures are cumbersome, time and energy consuming and expensive. In addition, the capability for soil water characteristics determination is generally lacking in Nigeria as only few laboratories have the required equipment such as tension table, pressure membrane apparatus and permeameter. Soil texture, on the other hand, is routinely determined in most soil laboratories in Nigeria. Thus, estimating soil water characteristics from soil texture will save time, energy and cost.
1.3 Project Justification
Estimating soil water characteristics from readily available physical parameters has been a long-term goal of soil physicists and engineers. This is because of the difficulties associated with the determination of the soil water characteristics. For instance, their determination usually involves soil sampling and laboratory work which are time consuming and laborious. In addition, the capability for their determination is generally lacking in Nigeria as only few laboratories have the required equipment such as tension table, pressure membrane apparatus and permeameter. Soil texture, on the other hand, is routinely determined in most soil laboratories in Nigeria.
1.4 Significance of study
The purpose of this project is to test the accuracy level of the Hydraulic Properties Calculator (HPC) model for predicting soil water characteristics in Niger State.
If this research shows that the program is applicable to the soil at the sampling sites located at Minna (Ferruginous tropical soil), Badeggi (Hydromorphic soil) and Mokwa (Ferrosol), which represent the three major soil types in Niger State, it can then be used for predicting the soil water characteristics of these soils and other similar soils in Niger State and by extension in Nigeria from their texture instead of separately determining the soil characteristics. However, a thorough evaluation of the model must be carried out before its widespread adoption in Niger State and beyond. The Hydraulic Properties Calculator (HPC) model (Saxton and Willey, 2006) to be evaluated is a graphic computer program used for estimating the water retention and water transmission characteristics of soil profile layers. Using soil texture modified by additional soil variables of organic matter, salinity, gravel and compaction, the program has the ability to predict soil water characteristics, namely wilting point, field capacity, bulk density, saturation capacity, plant available water and saturated hydraulic conductivity as output variables.
1.5 Scope and Limitation of the Study
The choice of the area of study for the research will cover Minna (Ferruginous tropical soil), Badeggi (Hydromorphic soil) and Mokwa (Ferrosol) locations in Niger State, Nigeria.
The results presented in the thesis were statistical average of many samples and therefore only an approximation of any specific soil layer status. The study is limited to one year duration between the month of June 2011 to July 2012.
1.6 Aim and Objectives.
The aim of the study is to establish the predictability and reliability of the model, and hence, its use in determining water characteristics of soils in Niger State. The specific objectives of the study were to:
1. Use SWC-HPC model to stimulate the soil water characteristics of soils in the locations under study.
2. To compare the model output parameters with those obtained from the laboratory test values for the same soils.
This material content is developed to serve as a GUIDE for students to conduct academic research
PERFORMANCE EVALUATION OF A SOIL WATER CHARACTERISTICS MODEL FOR THE THREE MAJOR SOIL TYPES INNIGER STATE, NIGERIA
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