Keywords

Wadis , spatial analysis , Remote Sensing , GIS , Libya

1 . INTRODUCTION

The study area located Northeast Libya, known as Al Jabal Al Akhdar (Green Mountain), as illustrated in figure 1, which is a crescent-shaped ridge reaching more than 875m above mean sea level in its central part as shown in the topographical profile (Figure 2). The Northern slope consists of two escarpments, divergent in the West but convergent gradually closer in the East direction; both escarpments are parallel to the Mediterranean seacoast. Also, the two escarpments dissected by several wadis, especially the second (Pallas, 1978). On the other hand, the Southern slope dips gradually toward the South. The hydrologic network Al Jabal Al Akhdar reflects the morphological features that divided into the Northern watersheds those flows to the North, which are short and profoundly cut and reach the sea after a few tens of kilometers, and the Southern watersheds those flows to the South, which are long, wide, and flat. They progressively disappear as valleys and become large spreading zones at the Southern breaking slope of Al Jabal Al Akhdar (Hamad, 2005).

Figure 1. Study area

Figure 2. North-South profile (SRTM data)

2 . STUDY AREA

2.1Geology

The exposed geological formation in the study area, mainly consisting of marine carbonate rocks from late Cretaceous age to Quaternary, as shown in the geological map (Figure 3). The late Cretaceous outcrops only distributed in the Northwest of the study area. While the Tertiary formations partially outcrop under thick, widespread Quaternary deposits cover in the whole study area. Based on a previous surface geological survey carried out by IRC in (1974), these formations described in summary as following:

Figure 3. Geology (IRC, 1974)

a) Late Cretaceous- Paleocene

The oldest formations outcropping in the northwest of the study area of Cenomanian to Paleocene (Danian) ages, where the following succession identified:

Qasr Al Abid Formation (Cenomanian)

Consist of greenish marl, glauconite, and pyrite, where only the upper part is outcropping with a thickness of more than 40m.

Al Baniyah Formation (Uppermost Cenomanian – Conacian)

It is represented by thick beds of limestone and dolomitic limestone intercalated frequently with marls and marly limestone in the upper part, with a thickness of more than 500m.

Al Majahir Formation (Campanian)

Consist of a sequence of marly limestone, microcrystalline limestone, and dolomitic limestone that interlayered with marl and calcareous clay, with thickness, ranges from 80 to 200 m.

Wadi Dukan Formation (Maastrichtian).

Consist of hard grey to brown dolomite crystalline to microcrystalline intercalated with porous dolomitic limestone in the upper part, with thickness ranges from 50 to 150m, which increase towards the Southwest of the study area.

b) Quaternary

The Quaternary deposits exposed in the whole study area, and consist of ancient and recent alluvial deposits.

2.2 Water Resources

The seasonal runoff in the wadis during the rainy seasons represents the surface water resource in the study area. Some of the runoff captured by water harvesting structures and used for domestic purposes, and some infiltrate into the groundwater, and the rest evaporates. Groundwater resource is the essential source for domestic and agriculture uses, where two main aquifers identified by (Franlab, 1978) and (GWA, 2006):

Upper cretaceous aquifer

The aquifer consists of dolomite and dolomitic limestone and characterized by a good quantity and quality of water, with thickness varies from 50 to 250m, distributed spatially in the Northern parts of the study area.

Tertiary Aquifer

The aquifer spatially distributed in the middle and southern parts of the study area and consists of carbonate rocks, dolomitic limestone, and dolomite in the Kharubah area and calcarenite limestone, while in the Makhili area consists of argillaceous limestone intercalated with evaporates, gypsum and halite.

Libyan secretariat of agricultural and land reclamation under the Executive Authority for Al Jabal Al Akhdar contracted with consultants to carry out several studies during the seventies. Frnlab French consultant carried out two phases of studies. The first phase was for collection, analysis, and interpretation of the preliminary data during 1974. The second phase carried out in (1976) the detailed study for groundwater by drilling exploratory wells and characterizing the groundwater in the region to determine the promising areas for groundwater abstraction for agriculture, where the irrigation scheme, economic feasibility, and soil the study conducted by C-Louti in 1977. Regarding surface water, Frnlab (1974) delineated the watersheds using the United States Geological Survey (USGS) topographic maps produced by in 1963, and conducting hydrological measurements during the period (1974-1976) leading to the identifications of the optimal locations for 27 selected dams in the area, those still not constructed yet. Moreover, Arlab Consulting Engineers contracted in (1982) with the Executive Authority for the supervision of drilling of production wells and piezometers and further groundwater studies. Also, the contract includes the complementary investigation of surface, groundwater, and climatological survey. Another work conducted by SWECO in (1986) regarding pasture land mapping for about 550,000 hectares. In 2004 several production water wells drilled for grazing and other purposes in the period of 2001-2007 (Hamad, 2008). The main focus for investment in water resources lies in the groundwater, which led to improper management and no property for the available surface water, which causes uncertainty for the surface water due to the scarcity of hydrological data. The exploitation of surface water is limited only to a few structures designed to collect rainfall from small catchment areas. These types of structures since ancient times built on moderate slopes or in low-lying positions in which the precipitation collected from catchment areas by a low rock or concrete walls which direct it into impervious underground storage compartments offered an essential source of water for human and livestock consumption. Although, many of these retention structures are old and some of them built more recently. The capacities of most of the structures are a few cubic meters. And recently, a few reservoirs of reinforced concrete constructed which are considerably larger, with capacities of up to 4,000 cubic meters. Also, the local communities made control of surface runoff water in wadis and depressions where cultivation practiced. The application is generally in the form of low retention walls or gabions built across wadis and depressions to give some control over the flood flows and improve water use for domestic use, agriculture and livestock. These structures result in larger areas being flooded for more extended periods, thereby increasing the area suitable for cultivation and improving yields, also reduce the water erosion.

Since the studies are scarce after the earlier mentioned work carried out in the area, therefore there is a need for essential knowledge background for the watersheds, which is crucial for better watershed planning and management. Furthermore, the assessment of the spatial characteristics is a cornerstone for the hydrologic studies and watersheds management. Therefore, this research aims to conduct a spatial study using the GIS techniques with RS data in order to delineate the watersheds and extract their spatial characteristics, land cover types, and the pattern of the principal climatic elements the precipitation and temperature.

3 . METHODOLOGY

The research methodology summarized as in figure (Figure 4) based on the data given in table 1. The watershed delineation conducted using the hydrology toolset in the software, ArcGIS Spatial Analyst extension. The tool offers elevation data processing to delineate watersheds, create stream networks through sinks identifications, and flow direction determination, and flow accumulation calculation, in addition to morphometric parameter computation and hydrologic analysis (ESRI, 2018). The hydrology toolset used the digital elevation model (DEM) data from Shuttle Radar Topography Mission (SRTM) available at the USGS Earth Resources Observation and Science (EROS).

Figure 4. Methodology

Moreover, the resultant features processed with spatial analyst tools in ESRI ArcGIS to extract their spatial characteristics. Land cover classification conducted using Landsat 8 OLI [Operational Land Imager] products that processed in Quantum GIS (QGIS) software using Semi-Automatic Classification Plug-in (SCP), which open-source plugin for QGIS that conduct semi-automatic supervised and unsupervised classification for remote sensing images. Also, it provides several tools for the pre-processing of images, the post-processing of the classifications, and the raster calculation (ESRI, 2018). The land cover layer produced according to step-B (Figure 4). The pre-processing was conducted by using DOS1 for atmospheric correction and supervised classification was performed using Maximum Likelihood (ML) classification algorithms. The accuracy was assessed by Semi-Automatic Classification Plug-in (SCP) accuracy tool with the help of ROIs using stratified random points. Also, the SCP accuracy tool provides the error matrix (Table 4).

Climatic data for the study area is very scarce, due to no periodical measurements, concerning some available measurements, conducted in short periods collected during the time of the earlier studies mentioned above. Therefore, this research uses the precipitation and temperature data from global climate monitor (GCM) which is a platform handle global climate data, where it offers the access to climatic information from 1901 to the date; also, it is an open-source project developed by the University of Seville Climate Research Group (GCM, 2019).

4 . RESULTS AND DISCUSSIONS

The result of the elevation data processing, which is illustrated in figure 5 and the spatial characteristics in the table 2 and table 3. The overall areas of the watersheds are 17721 km². The hydrologic divide, which that bound the watersheds in the North delineated, where its length computed as 47075km, extends from the Southwest to East. The delineated watersheds classified based upon the areal extent according to (Singh, 1994) as basins excluding Thuban, which considered a watershed. The slope in the Northern parts, which is the head of water zones, prevails moderate slopes, while in the middle, the transitional zones prevail low slope values and very low in the deposition zone. The aspect ratio for all the watersheds is facing Southeast direction, except Muallq and Tamimi watersheds those facing East directions. Also, it was evident from the layout of the watersheds the influence of the upper Cretaceous and Tertiary tectonic events that were dominant in the study area, in addition to the impacts of the climatic processes, which induced the weathering, and erosion processes of the geologic formations.

Figure 5. Delineated watersheds of the southern Al Jabal Al Akhadar

Furthermore, stream lengths calculated for all the watersheds, which are 35757.8km. The streams in the study area can be defined according to (EPA, 2008) as ephemeral those flows shortly due to the direct response to rainfall events in the immediate surrounding area and its channel always above the groundwater aquifer. Moreover, a large number of wadis in the upstream of the watersheds are remarkable, which decreased southward and become braided in some localities in the midstream, and downstream. The outlet of the wadis mostly in the temporary lakes, those locally known as balat, which are topographically flat depressions areas located downstream. The spatial characteristics of the balat are shown in table 3, where the balat are vital areas for grazing, in which the surface water during the rainy season harvested by manmade trenches which is one of the prevailing water harvesting techniques in the region for collecting and storing water during the rainy seasons to be used for watering during the dry periods, as in the figures 6 and 7 and the trench generally about 1km long with 40m wide, and reaches about 5m in depth in the middle.

Figure 6. Mehrez temporary lake (Google Earth image, 2019)

Figure 7. Aerial view of manmade trench in Balat Mehraz (Google Earth image, 2019)

Land cover classification processed for accuracy as in table 4 and the land cover map produced (Figure 8), also the area of each land cover class computed, as shown on the table 5. The land cover consists of five classes which can be described as in the following:

Figure 8. Land use land cover

Alluvial plains

Flat areas formed as a result of erosion due to weathering and runoff, as the sediment from the upstream, transported to the lower plain composed mainly of ancient alluvial deposits that consist of silt, sand, clay and gravel.

Spreading zones

Mostly located on the banks of the wadi channel and consisting of recent alluvial sediment of silt, sand, clay, and gravel.

Forest

Forest consists of dry Mediterranean woodlands, which limited only to the Northern parts of the study area and consist mainly of cypress Juniper (Juniperus spp.), Cypress (Cupressus spp.), Pinus (halepensis spp.) and Ilex (Ilex spp.) and other trees.

Grassland and steppes

It consists of plant communities of dwarf shrubs, grasses and annual herbs sparsely distributed in the study area.

Bare rock

Bare rock consists of consolidated and non-consolidated marine carbonate formations that range in age from late Cretaceous to Quaternary.

Climate in the study area classified as a semi-arid climate in the North and as an arid climate in the South (Hamad, 2008). Average annual precipitation from 2008 to 2018 (Figure 9) shows the highest rates of rainfall in the Northern parts that gradually decreased to the south direction, while the precipitation mainly occurs from October to March and April. Also, the distribution and amount of rain showed great differences between years; during the rainy season, sudden and robust runoff flows to the temporary lakes. The runoff is mostly lost by the evaporation due to higher temperature (Figure 10), the mean annual temperature, which shows the increases of temperature southward. The coldest January temperature is 8-12°C. The highest temperatures occur from June to August, with maximum temperatures over 40°C. The mean July temperature is 22-26°C.

Figure 9. Mean annual precipitation (2008-2018)

Figure 10. Mean annual temperature (2008-2018)

5 . CONCLUSIONS

The South of the Al Jabal Al Akhdar area assessed spatially, where the results show large and wide watersheds with low and moderate slopes. The land cover assessment shows only the Northern parts covered with forest, while the most are barren lands intercalated with alluvial plains, spreading zones, grassland. The watersheds receive a considerable amount of precipitation, which small quantities harvested or naturally recharged to the groundwater aquifers. Unfortunately, the rest unallocated and appropriately invested, as most of the rainfall lost by evaporation because of the high-temperature in the South parts of the study area. Moreover, the remain of runoff goes to the sudden floods that occur annually leading to natural disasters that cause human and economic losses, in addition to soil loss by erosion and transport from the North to the South as its noticed from the land cover map, where this loss will lead to degradations in the Northern parts of watersheds. Finally, the study area very vital, especially for pastoral activities, also promising in surface water investment rainfed agriculture, but the watersheds require further detailed studies for the morphometric and hydrological characteristics. Although these studies rely on data, which are scarce in the study area, earth observation satellite data are adequate with the support of ground-truth.

Tables

Figures

Conflict of Interest

Acknowledgements

Abbreviations

References