Impacts of Watersheds’ Landscape and climate descriptors on surface runoff in moun- tainous region

Abstract: Watershed’s landscape features and climate variables have a significant influence on the mountainous catchment’s hydrological response. This literature review synthesizes recent kinds of literature investigating associations between surface runoff and catchment’s landscape features, and the potential controls of climate variables, with an emphasis on mountainous regions. Such factors are significant controls on surface runoff through their influence on the rate of infiltration capacity, antecedent soil moisture conditions, and underlying bedrock structure. The literature review indicates that there are considerable issues that remain to be resolved in advance a concrete understanding of the influence of catchment’s characteristics on surface runoff response.


Introduction
"Mountainous region plays a crucial role in the supply of freshwater to humankind, in both mountains and lowlands." [1] In a mountainous region, a scientific understanding of catchment processes and surface runoff is very essential to effective water policy, development, and management. Urbanization and population evolutions over time are associated with highly increasing demands on accessible freshwater for different purposes like human consumption, industry, agriculture, and environmental regulation [2]. In this region, surface runoff generation and its relationship with climate-landscape features have considerable influence on phenomena like soil erosion, land degradation, desertification, and flooding that impose significant impacts on a community [3]. Confirming save concentrations of pollutants associated with agricultural activities such as fertilizer doses and pesticides, chemicals that are used to kill pests and weeds, requires accurate estimation of surface runoff volume or depth since highly polluted overland flow may be drained into streamflow [4][5][6], and these factors transport negative implications for stream biota and human consumption if it is beyond the limits of WHO [7,8].
In the mountainous area, soil erosion phenomena due to surface runoff are highlyprone (e.g.[9]), and this erosion resulted in both 'on-site' and 'off-site' problems. On-site problems like a significant loss of agricultural soil fertility on the upper part of the watershed at a rapid rate, resulting in a great loss of natural resources and agricultural yields [10]. Contrary to this, spring development in the mountainous regions can be successfully applied if a watershed is conserved and protected from the unwise use of human beings [1]. Off-site problems such as sedimentation which affects the life span of the reservoir as well as streamflow due to loss of its storage, [11,12], and water quality [13,14]. [15] investigated that, about 10 out of the 15 reservoirs in Jamaica were significantly silted. Supplement to this, the environmental impacts of sedimentation such as loss of aquatic habitat, decreases in fishery resources, and loss of wetlands [16,17]. Accordingly, a stronger knowledge on the surface runoff is well fundamental in different issues such as non-point contaminant pollutions [18], and in the water scarcity regions like arid and semi-arid mountainous regions, even in a humid mountainous region, areas where water shortages are not communal, the overland flow may be regarded as an additional water resource source [19].
Even if the necessity for a better understanding of surface runoff response to external factors has been known for many years, most previous reviews have tended to emphasize flood response to increased human pressures on the catchment's landscape [20,21], and some researches focus on investigating the baseflow response to catchments' landscape and climate variables [22,23]. In this regard, the literature review is inadequate concerning researches exploring mountain area surface runoff response to biophysical natural factors like geology, relief, forest, climate, soil, and human influences on watershed's landscape.
The purpose of this review was to synthesize investigation from various water resources disciplines, to provide an organized summary of the current state of research knowledge regarding the influences of watershed characteristics and anthropogenic influences on surface runoff, and to address the potential impacts of climate variables and their changes on surface runoff in a mountainous region. Water resource management involves a stronger understanding of surface runoff processes, and a secondary objective of this review is to identify key research questions as a gap that remains unanswered.
This review stresses literature covering geomorphic and anthropogenic effects including climate on surface runoff in mountainous regions of the world. The introductory part covers a basic discussion of primary controls on surface runoff as well as its impacts on human beings, and environmental services. Next, a section on geomorphic controls on overflow processes covers the influences of basin geology, surface topography, and soils. This section is followed by an overview of the anthropogenic effect on surface runoff, with emphases on deforestations, agricultural activities, and urban expansion, because of the large body of research on those topics. Next to this, a summary of current research evaluating and predicting surface runoff response to climate variables and their change is presented. Finally, the review concludes with a discussion of key research topics, the results of which would fill large gaps in our understanding of watershed hydrology and surface runoff.

Surface Runoff Overview
As a result of either the rainfall rate exceeds the infiltration rate or soil water holding capacity exceeded, water runs off along the land surface and/or hillslopes as overland flow or surface runoff (Horton, 1933(Horton, & 1938   The saturation excess overland flow is more common in a mountainous region under temperate and/or humid climate conditions with frequent, low precipitation intensity, and occurs when the soil is saturated and unable to absorb more water [26]. Figure 2 summarizes the fundamental processes involved in runoff generation, indicating the interaction between infiltration excess, saturation excess, and subsurface flow pathways. Several rainfall-runoff models are organized around a representation similar to Figure 2 involving the partition of surface water input into infiltration or overland flow, either due to infiltration excess or saturation excess. Factors that encourage infiltration, percolation, and higher evapotranspiration (ET) will decrease surface runoff, while factors associated with anthropogenic activities like urbanization and logging will increase surface runoff.
Overland flow is influenced by a wide range of features [27-30] like: • Geomorphology of the catchment's landscape; • Spatially distributed watershed's soil characteristics; • Land use land cover change throughout the catchment; • Catchment's geologic characteristics.
Most of the physiographic features may be changed with anthropogenic effect on the catchments' landscape, and it therefore very essential to understand not only the associations between catchments' landscape characteristics and surface runoff in a mountainous region but also how direct anthropogenic watershed impacts and climate change affects these physiographic characteristics.

Geomorphic Controls on Surface Runoff
In a mountainous region, surface topography contributes a significant role in geomorphological and hydrological processes [1,30], specifically, surface topography plays a significant control in surface runoff either directly, or indirectly [31]. Topographic slopes control the rate at which surface runoff moves downslope, thus determining whether an overland flow is drained into the stream channel network or retained in the soil [32]. In explaining the role of topography, several topographic indices (TIs) have been established and used to support understanding hydrological processes (e.g. surface runoff peak, baseflow, and low flow), and to explain the distinction between catchments [30]; [33]. Even though the influence of topography in controlling numerous flow magnitudes has been extensively studied [33], the numerical association between specific TIs and various flow variables (e.g. overland flow, baseflow and/or low flow, and peak flow) is not well understood [30].
[34] tried to classified Topographic indices (TIs) into two groups, that is, primary and secondary indices). Primary indices (e.g., slope, elevation, drainage density, and aspect) are usually directly measured from a digital elevation model (DEM), whereas secondary indices were calculated of primary indices that are used to describe the role of topography in geomorphological, biological, and hydrological processes. For example, the topographic wetness index (TWI) is defined as ln( / tan ), where α is the upward slope contributing area per unit contour length, and β is the slope angle at that specific area [30].
According to [35], steep hillslopes in mountainous catchments were highly susceptible to surface runoff, which resulted in high sediment yields.
[21], took a study on Dire Dawa City, Ethiopia on flood risk analysis, and they found that Dire Dawa City was circumscribed by various chained mountainous area like Dengago, Kersa, Kulubi, and Meta Mountains, as a result, a huge surface runoff drained from those mountainous regions and inundated the city. Table 1. . summary of topographic features and their correlations hydrologic response (source: [36]; [37]).  (Table 1). To this end, it is very important to note that recent done and ongoing research shows that variation in GIS-based processed digital elevation model (DEM) resolution (e.g. 1km, 90m and 30m resolution) can have a significant effect on rainfall-runoff hydrological analysis, and more scientific-based investigation requests to be conducted to associate DEM-based topographic features with the overland flow at multiple resolutions [38].

Soils Characteristics
Soil properties and their rate of formation are highly dependent on underlying bedrock geology [39], and topographic position, which affects the hydrologic response of a catchment like precipitation infiltration capacity, percolation, overland flow, subsurface flow, and soil moisture storage. Variation in soil texture and/or soil profile along the hillslope of mountainous catchment plays a significant role in the rate of surface runoff [40]. Additionally, surface runoff is extremely affected by spatial and/or temporal variability of soil initial moisture retention, which may be predominantly controlled by surface and/or subsurface topography [41], and in part soil texture, which affects the saturated hydraulic conductivity of the surface soil (Ks) [40,41].
Consequently, in a mountainous region, the associations between soil initial moisture retention and hillslope position are expected to exist, with very small particle size, denser soils, and low slope gradients merging their effects to encourage soil moisture holding, and most probably saturation-excess surface runoff occurs. On the contrary, steep upper hillslopes, soils are likely characterized by coarser, less developed, and thinner as soil accumulation is strongly limited due to strong both wind and water erosion [42], thereby more rapidly transmitting surface runoff water, as the result infiltration excess runoff most commonly happens [43]; [44,45] (Figure 3). According to [46], landslides due to soil saturation and perched groundwater dynamics can cause flash floods and mudflows when severe rainstorms occur on steep hillslopes with shallow soils. From this Point of view, separating the influence of soil characteristics from topography in a mountainous region on a hydrological response, specifically on surface runoff is yet a challenging task, which needs further research.

Human Being Land Uses Control on Surface Runoff
In a mountainous region, extensive land-use changes, and soil disturbance go together with most forms of land-cover change [47], and such influences are usually sufficient to modify the timing and quantity of surface runoff [48,49]. Furthermore, the anthropogenic impact may involve direct removal of forests (deforestation) through cutting and/or wildfire, and urbanization [47]. This sub-section synthesized different literature review on anthropogenic controls such as forest removal and/or plantation, urbanization, and agricultural activities on surface runoff.

Vegetations
In the forested mountainous region, vegetation type and cover have spatial and temporal variation, and as a consequence, this vegetation variance might have significant control on runoff generation and transfer mechanism [32,33], and specifically in a forested humid region, Horton overland flow generation is rare [50]. On the contrary, in arid and semi-arid mountainous regions with deforested and/or scarce vegetated catchments, Horton surface runoff generation is the most dominant processes (Figure 3).
[35] took a study on Lake Hayq catchment, South Wollo mountainous region, Ethiopia to detect landuse/land-cover change over 50yrs using multitemporal remote sensing and geospatial data, and they found that farmlands and shrublands were increased, whereas the bushlands, grasslands, forestlands, and Lake surface area were reduced over the past 50yrs, resulted in accelerating soil erosion in the basin, and sediment accumulation into the lake.
However, the study didn't consider the rest landscape descriptors like soil characteristics, underlying bedrock geology, and topography of the Lake Hayq catchment. [51] took research on East Africa Region, and estimated that due to forest loss annual discharge and surface runoff increases by 16±5.5% and 45±14% respectively.  [32]. [53], were simulated and estimated the surface runoff increases by 5.7% as a result of deforestation in Suiá-Miçu River basin, Brazil, and the same result was performed and confirmed in 27 catchments in South East Asia due to forest cover loss [54].

Urbanization
Following urbanization, impervious surface coverage, compacted soils, and open dumping and roads [59], and complex pollutants like microbial pollutants [60,61], synthetic chemicals [62,63], pesticides [64,65], and pharmaceuticals [66,67]. According to [57]; increasing impervious surface coverage and soil compaction following urbanization would be linked with corresponding decreases in recharge in the urban system, as a result, a negative effect on baseflow quantity and quality. Furthermore, a series of studies found that compacted urban soils can preclude or slow natural processes like infiltration and throughflow, resulting in increased surface runoff [68,69].

Agricultural Activities
Overland flow response of a mountainous catchment is highly influenced by several land-use descriptors, and among them, agricultural activities are the one [57,70]. Low valley areas of mountainous regions are relatively flat and suitable for agriculture, and in connection with this unwise manipulating various agricultural activities resulted in a loss of fertile soil due to surface runoff. [71] investigated that along hillslope, agricultural fields have generally lower canopy density than the natural vegetation, which makes them more vulnerable to surface runoff and nutrient flushing. Farmland plowing in particular breaks up and make softer the structure of soils and serves as a major contributor to surface runoff-based soil erosion [14], and overland flow-based severe soil erosion which leads to excessive sediment export to streaming and/or reservoirs results in disturbances of life in water bodies as well as reduced duration/life of reservoirs and affects the quality of the environmental services [72]. According to [33] investigation, surface runoff in agricultural fields reduced infiltration rate and decreased baseflow.

Geology
Weathering is a process of change and fragmentation of rocks by various natural events like physical, biological, and chemical processes [73]. In a mountainous region, a catchment's spatial characteristics of the geological structure and underlying bedrock

Climate
Global and regional climate components, and their change caused by a human being and natural events bring influences on catchment's hydrological processes specifically on surface runoff processes [79]. For instance, [80] confirmed that global average temperature was projected to remain steady as a result of constantly increasing atmospheric greenhouse gas concentrations, resulting in a significant influence on runoff generation [81].
[82] studied selected five catchments, which were located in southwest China using a partial least squares regression (PLSR) model to detect the dominant climatic variables driving extremes on annual surface runoff, and results indicated that annual total wet day precipitation (PT), rainy days (RDs), heavy precipitation amounts (R25), heavy precipitation days (RD25), rainstorm days (RD50), and rainstorm amount (R50) are dominant deriving factors of annual surface runoff.
The expected global climate changes that will touch the mainstream of the world may involve some combination of temperature increase and either precipitation increase or decrease, as a result, overland flow response to climate change will depend on the magnitude and direction of changes in both precipitation and temperature [83,84]. A further substantial difficulty to understand the influences of climate variables, and their change on overland flow is that experimental researches assessing surface runoff response to changing climate variables naturally are unable to clearly demarcate from coexisting landuse change during the period of assessment because of uncertain interactions of factors driving these changes [51], and a better understanding of the controls of climate changes on surface runoff is indispensable for mitigation of natural hazards like flooding [82].

Conclusions
From the beginning, knowing just how catchment's landscape structures and climate variables will affect surface runoff processes, in the framework of catchment geomorphology, will support watershed managers and environmentalists in the protection and conservation of mountainous catchment, which is the headwater of all streams. For instance, in the mountainous region, accelerated runoff increases the impact of soil erosion, which affects the life of downstream reservoirs due to sedimentation [85], reduces soil fertility [86,87], and the capacity of soil water storage [88], and this may lead to reduced vegetation cover and productivity, forcing people to intensify land use land cover change.
The close linkage between climate variables like precipitation and catchment's hydrological response such as annual surface runoff was determined by partial least square regression (PLSR) approach, and consequently can provide useful and quantitative information that enables decision-makers to make better decisions concerning water resources management [82].
This review has revealed that watershed's topography and geology influences surface runoff response by affecting the soil water holding capacity properties, overland flow generation mechanism, and infiltration and/or percolation rate within a watershed.