Assessment of land use / cover impacts on runoff and sediment yield using hydrologic models : A review

Human activities have modified the environment over the years. Urbanization, agriculture lumbering, mining and other land uses have substantially altered the Earth’s surface. Land use and the resultant change in land cover have significant effects on ecological, environmental and hydrologic systems and processes. An understanding of past and present land-cover change, together with an analysis of potential future change, is necessary for proper management; thus, the need for models. Hydrologic models are primarily used for hydrologic prediction and for understanding hydrologic processes. With recent technological advances, technological based tools such as GIS are incorporated into hydrologic models for assessing the impacts of various land use/cover. Hydrologic models incorporated with GIS can be used to project future land uses/cover to provide an increased clarity, probability or likelihood of potential consequences on ecosystem services such as biodiversity, water quality and climate. This paper critically examines land use/cover, effects of impacts of land use/cover and the use of hydrologic models to assess the impact of land us/cover on runoff and sediment yield. Hence it calls for their use by watershed managers and decision maker as management tool especially in developing countries.


INTRODUCTION
Human activities have modified the environment over the years.The world has changed dramatically especially after the industrial revolution.While Earth's landmass has remained essentially static over that time, the human demands on it have grown and changed, impacting the land and its flora and fauna in numerous ways.Land use change in Africa included the conversion of 75 million hectares of forest to Agriculture and pasture between the years 1990 and 2010, a rate second only to that in South America (FAO, 2010).
Rapidly changing human activity within the natural environment can put huge pressures on the natural environment's ability to adapt and change.These may be further complicated by the influences of climate change, such as extremes in weather.Maintaining a balance between urban development and natural systems is essential to a safe ecosystem.Agriculture and urbanization are major forms or drivers of changes in land uses/land cover (Fisher and Unwin, 2005).Throughout history, agriculture has had a significant effect on the world's landscape.Agricultural production has caused greater environmental change to the biosphere *Corresponding author.E-mail: emeka.ndulue@unn.edu.ng.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License than any other land use (Gliessman, 1998).Until the industrial revolution of the early to mid-1900, farming practices were relatively environmentally friendly.The modernization of farming practices around the 1950's resulted in extreme increases in productivity often to the detriment of environmental quality.These conventional agricultural practices, however, have numerous long-term ecological impacts such as soil degradation, habitat alteration, water quality impacts, species composition impacts and adverse effects of irrigation.
Urbanization is another major driver of land use/cover.Urban population has been increasing significantly in the last two centuries, since industrial revolution took place.The consequences of this process in curs great changes of the natural environment.Urbanization process tends to substitute natural vegetation for impervious surfaces, thus reducing infiltration.It also tends to eliminate natural detention ponds, to rectify river courses, among other actions, that greatly interfere with superficial flows (Miguez and Magalhaes, 2010).The conversion or transformation of land uses from one form to another has a resultant effect on the ecosystem, which may be immediate or remote.
Conversion of agricultural, vegetation and wetlands to urban areas and the unattended population growth usually come with a vast increase in impervious surfaces, consumption and utilization of goods, and building on natural drainages (USEPA, 2001;Ifatimehin et al., 2009).
The hydrologic cycle involves complex interaction and processes among climate, landuse, vegetation cover density, erosion rates and sediment loads in watershed.The complexity and uncertainty in natural systems like hydrologic cycle make them difficult to understand, predict and manage.The need for more scientifically sound analyses has led to the development of hydrologic models.Hydrological models provide a framework to conceptualize and investigate the relationships between climate, human activities (e.g., land use change) and water resources (Legesse et al., 2003).
The resultant effects of these land changes and transformation can be classified into ecological and environmental, hydrologic and socio economic.This paper examines the different landuses, the consequences of land use/land cover changes and evaluates the use of hydrologic models in determining the impacts of landuse/cover on runoff and sediment yield.Such hydrologic models include SWAT, WEPP, AnnAGNPS, TOPMODEL, MIKE-SHE, DRAINMOD, etc.

DEFINITION OF LAND USE AND LAND COVER
The terms land cover and land uses are often confused and used inappropriately.Land use can be defined as a series of activities undertaken to produce one or more goods or services.Hence, land use is based on function, the purpose for which the land is being used (FAO, 1997).IPCC (2001) defined the term land use to cover the entire range of direct management activities that affect agricultural soils, result in land-use change, alter forest management, or affect the long-term storage of carbon-containing products.All such activities are implicitly human-induced.Examples of land uses are agriculture, forestry, recreation, etc.
Land cover is the observed physical cover, as seen from the ground or through remote sensing, including the vegetation (natural or planted) and human constructions (buildings, roads, etc.) which cover the earth's surface (FAO, 1997).Water, ice or sand surfaces are examples of land cover.
This means that the cover on a land points to the kind of activities or uses on the land.For example, agricultural practices are usually carried out in a forested or vegetated land, while an urban area is usually filled with impervious area due to vegetation removal.Land cover information is captured using field survey or analysis of remotely sensed imagery.Land cover maps provide information to help managers best understand the current landscape, assess urban growth, model water quality issues, predict and assess impacts from floods and storm surges, track wetland losses and potential impacts from sea level rise, prioritize areas for conservation efforts, and compare land cover changes with effects in the environment or to connections in socioeconomic changes such as increasing population (www.http://oceanservice.noaa.gov/landuse,2009).To see change over time, land cover maps for several different years are needed.With this information, managers can evaluate past management decisions as well as gain insight into the possible effects of their current decisions before they are implemented.
Figure 1 shows data on land use and population change for regional (Africa and Middle East) and global scale in the last 300 years for three main land use types.Figure 1a and b depict these changes.From Figure 1, it is seen that as population increases, there is an increase in cropland and grassland land uses while there is a decrease in forest land cover.These changes in land use/land cover have effects on the ecosystem.

LAND COVER/LAND USE CLASSIFICATION SYSTEM
Many classification systems are being used throughout the world.However, there is no single internationally accepted land cover classification system (FAO, 1997).Different organizations set up their classifications differently, because they are interested in different aspects of land use and land cover (CARA, 2006).General constraints for building land cover/land use classification are linked with general constraints of nonoverlapping and completeness, textural rules and specific constraints linked with time of observation and data collection (Duhamel, 2012.).However, rules for land use/land cover classification system can be obtained in Duhamel ( 2012) and (FAO, 1997).
Many land use/land cover classifications are based on a system developed by Anderson et al. (1976).Anderson's (1976) classification combines information on land use and land cover, placing all land into one of 9 level-I categories: Anderson level-I categories 1. Urban or built up land; 2. Agricultural land; 3. Rangeland; 4. Forest land; 5. Water; 6. Wetland; 7. Barren land; 8. Tundra; 9. Perennial snow or ice.
Subcategories make finer distinctions.For example, level-I category 1 (urban land) could be divided into level-II subcategories such as: One possible set of level-II subcategories 11.Low density residential; 12. Medium density residential; 13.High density residential; 14.Commercial; 15.Industrial; 16.Institutional; 17. Extractive; 18. Open urban land, including parks and golf courses.
Each of these subcategories also can be divided.For example, level-II subcategory 14 (Commercial) could be divided to distinguish between office buildings and shopping malls, or to distinguish among commercial buildings associated with different industries (retail, health care, etc.).

EFFECTS OF LAND USES/COVER CHANGES
Conversion of a land cover has its accompanying effects and impacts, of which in most cases negative and detrimental to the ecosystem.Analyzing land cover change is important because surface changes affect a wide variety of ecological processes.Hence the effects of land use/land cover are broadly classified into ecological and environmental, hydrological and socio economic effects.

Ecology and environmental impacts
The impacts of land use changes have received considerable attention from ecologists, particularly with respect to effects on aquatic ecosystems and biodiversity (Turner et al., 2001).According to Wu (2008), land use change is arguably the most pervasive socioeconomic force driving changes and degradation of ecosystems.Briassoulis (2013) classified the environmental impacts of land use/land cover at large (global) scale, regional scale and local level.
At global scale, environmental impacts include land degradation and desertification, biodiversity loss, habitat destruction and species transfer.Species such as the Upland Sandpiper have drastically declined in regions where native grasslands have been lost (Kirsch and Higgins, 1976).
At regional level, the environmental impacts of land use change are equally significant and felt.Its impacts include eutrophication of water bodies, acidification of aquatic and terrestrial ecosystems, floods, soil nitrate pollution, land degradation and desertification, groundwater pollution, marine and coastal pollution and many more are environmental alterations that follow either directly or indirectly from land use changes.
Finally, at the lower spatial level, which is mainly caused by urbanization, industrialization and development, land use/land change impacts include changes in the hydrological balance of the area, increase in the risk of floods and landslides, air pollution, water pollution, etc.Others are soil erosion, sedimentation, soil and groundwater contamination and salinization, extinction of indigenous species, marine and aquatic pollution of local water bodies, coastal erosion and pollution.

Hydrology
Land use change and cover have a strong impact on water resources both in terms of their quantity, quality and increased variability of hydrological components like rainfall, etc. Land-use change alters runoff patterns, change stream flows, and increase the likelihood of flood events.Land use changes in a watershed can impact water supply by altering hydrological processes such as infiltration, groundwater recharge, base flow and runoff.
For instance, converting a forested watershed to a commercial or highly densely populated area may results in increased surface runoff and surface erosion rates.

Socio-economic impacts
In classical economics, land is one of the factors of production.Hence, land use is the backbone of agricultural economies and it provides substantial economic and social benefits (Wu, 2008).Land use change, however, does not come without costs.For instance, conversion of farmland and forests to urban development reduces the amount of lands available for food and timber production.Also, it may lead to reduction in land quality through soil erosion, salinization, desertification, and other soil degradations.Also, the conversion of one land cover to another diminishes the aesthetic value of nature.

APPLICATION OF HYDROLOGIC MODELS ON THE IMPACTS OF LAND USE CHANGE ON DISCHARGE AND SEDIMENT YIELD
Hydrologic models are simplified, conceptual representations of a part of the hydrologic cycle.They are primarily used for hydrologic prediction and for understanding hydrologic processes.Developments in computer technology have revolutionized the study of hydrologic systems.
An integrated landscape model can potentially extrapolate from management practices and land use pattern to determine potential environmental impacts (Turner et al., 2001).The usefulness of hydrologic models for environmental management is explained with a focus of prediction uncertainty.The prediction ability of these models makes them suitable as management tool for planning and decision making in our watershed.Thus, the development of an integrated approach that can simulate and assess land use changes, land use patterns and their effects on hydrological processes at the watershed level is crucial to land use and water resource planning and management (Lin et al., 2006).Numerous studies have developed modeling approaches to simulate the pattern and consequences of land use changes.Different types of models are used to explore land use changes.A review of some hydrologic model in this study includes SWAT, WEPP (GEOWEPP), AnnAGNPS, DRAINMOD, MIKE-SHE and TOPMODEL.Table 1 shows the application of some hydrologic model on different land scenarios and the results showing the impacts of land use change scenario as predicted by the hydrologic models.

Model performance
Hydrological models are usually evaluated using statistical analysis.They show relationship between simulated or predicted values and measured or observed value.They tell us how well the hydrologic model predicts or performs in simulating a process.The evaluation of hydrologic model behavior and performance is commonly made and reported through comparisons of simulated and observed variables (Krause et al., 2005).In this review, models were evaluated using Pearson's correlation coefficient (r) and coefficient of determination (R 2 ), Nash-Sutcliffe efficiency (NSE), Percent Bias (PBIAS) or Relative Errorand Root Mean Square Error (RMSE).
Pearson's correlation coefficient (r) and coefficient of determination (R 2 ) describe the degree of collinearity between simulated and measured data.Correlation coefficient ranges from -1 to 1 while coefficient of determination ranges from 0 to 1.

CONCLUSION AND RECOMMENDATION
As the global human population grows and its consumption patterns change, additional land will be needed for living space and agricultural production.This will result in land use/and cover change.It should also be noted that changes in land cover and land uses has it attendant problems and effects; hence the need for proper management.However, in trying to know how different changes in land uses and cover will work, models can be employed.Models are used to predict or forecast future configurations of land use patterns under various scenarios.Hydrologic models can play an instrumental role in impact assessment of past or future activities in the environmental and/or the socio-economic spheres.Developments in computer technology have The results from the scenario analysis of land use change showed that the outflow was affected by converting forested land to cropland, and proportionally increased with an increase in the proportion (0.0-1.0) of cropland area at an average rate of 0.3 from MIKE HE and 0.35 from DRAINMOD during the three year period.Also, annual outflow can be increased by 64-69mm for a conversion of the forested land in the uplands on the watershed to cropland, and by 113-122 mm for a complete watershed conversion.The results indicated that the highest sediment yield per unit area produced from agricultural lands (23.95ton/ha/yr), and followed by rangelands (4.69 ton/ha/yr) and forest lands(1.32 ton/ha/yr).All scenarios' simulations resulted in a decrease of soil losses and sediment yield comparing to the current land use status.Also, results showed that cultivation of pasture with forest-mixed resulted in the highest mean annual reduction in sediment yields (-6.08%), and 8.31% increase of stream flows in dry season.The result shows that all three scenarios resulted in an increase in discharge during wet months and a decrease during dry periods.The deforestation scenario was the one that resulted in the greatest modification of total monthly runoff.Simulations using SWAT indicated that conversion of corn-soybean to corn-cornsoybean would cause 11 and 2% increase in sediment yield and TP loss, respectively.The conversion of corn-soybean to continuous corn caused 55 and 35% increase in sediment yield and TP loss, respectively.revolutionized the study of hydrologic systems.Many computer models have been developed for hydrologic modelling and water resources management applications.However, hydrologic models can be used to prescribe optimum patterns of land use for sustainable use of land resources and development.This gives us the predicted effects and impacts of a given land use using different scenarios.Hence, it is a recommended tool for proper watershed management, especially for developing countries.

Table 1 .
Application of hydrologic models used to predict land use/cover change effects on discharge and sediment yield in different watershed across the world.