Distance Learning Module 2
Distance Learning Module 2 - The Hydrologic Cycle in Southwest Asia
A. Water sources in atmosphere in Southwest Asia
- Westerly winds and frontal storms (from Atlantic Ocean and Mediterranean Sea).
- Southern Monsoon winds and storms (from Indian Ocean) (Figure 2.1).
Figure 2.1. Seasonal maps of Asia showing dominant wind directions and positions of the intertropical convergence zone (ITCZ) of the stormy, moisture-laden air masses that rise because of their warmth and cause monsoonal precipitation. The thermal low pressure in the summer season over Pakistan and part of Afghanistan shows the patterns of winds in the region at that time. The Asian monsoon occurs where winds reverse direction throughout the year to move into the intertropical convergence zone (ITCZ) of low air pressure caused by hot heat rising from the sun’s summer heat. The ITCZ moves back and forth across the Equator (0° latitude) from June – July to December – January. Not shown are the winter storms coming from the west into Afghanistan and Pakistan with moisture derived from the Atlantic and Mediterranean seas.
B. Wind sources
Westerly winds from Atlantic & Mediterranean in winter.
Monsoon winds from south Arabian Sea & Indian Ocean in summer.
Wind of 120 days from north and west in Afghanistan in summer into monsoon low pressures dominantly in Pakistan (Figure 2.2).
Figure 2.2. In the Afghanistan – Pakistan region during the summer the ITCZ is located on average approximately along the border between the two countries. Strong winds from Central Asia are drawn south and east across the western lowlands of Afghanistan toward the low pressures of the ITCZ as the wind of 120 days (badi sado bist roz). In some cases the monsoon rains can fall as far north as the Hindu Kush, but when that occurs over the overgrazed and deforested mountains, the rapid water runoff forms rapid, wet, debris flows of mud and rocks that race down river valleys as a serious flood and landslide hazard. Figure 2.3 Cold descending katabatic winds (upper) and warm rising anabatic winds that are typical in mountainous desert areas. Katabatic winds move down from cold highlands and anabatic winds move up from hot lowlands. Both kinds of winds occur in Afghanistan, Tajikistan, and Pakistan.
Katabatic winds – cold air drainage down from Tibetan Plateau & other high ground.
Anabatic wind – hot air rising up from low-land deserts (Figure 2.3).
Figure 2.3. Cold descending katabatic winds (upper) and warm rising anabatic winds that are typical in mountainous desert areas. Katabatic winds move down from cold highlands and anabatic winds move up from hot lowlands. Both kinds of winds occur in Afghanistan, Tajikistan, and Pakistan.
Orographic winds – where air with water vapor in it is forced up over mountains the air cools and water condenses out as rain. Then on the other side of the mountain the air warms up and dries as it moves down again into the rainshadow (Figure 2.4A).
Figure 2.4A. Orographic rainfall that rains out on one side of a mountain and then the descending air on the other rain-shadow side warms and drys.
Heating winds – where the Sun heats up the ground the air warms and rises which pulls in other air (Figure 2.4B).
Figure 2.4B. Heating winds may be drawn in from other places when the very hot air rises.
C. Precipitation in Afghanistan
Precipitation in Afghanistan is greatest in the higher northeastern parts of the country where a combination of Westerly Winds, orographic precipitation, and monsoon winds and precipitation bring in the most moisture. The intersections and interactions between all these winds produce complex weather and climate patterns that can be affected adversely by climate change now underway (Figure 2.5).
Figure 2.5A. Map of average annual precipitation in Afghanistan and nearby countries.
Figure 2.5B. Monsoon air and precipitation arrive in Kabul sometimes.
Figure 2.5C. Temperatures of air with altitude, and different wind types (monsoon flow, antimonsoon flow, subsiding continental air, planetary Westerly flow).
D. Snow & glaciers in mountains
Snow in mountains that doesn’t melt away each summer will change to glacier ice in 5-7 years. Glacier ice moves downhill because of gravity (Figure 2.6A&B).
Figure 2.6A. Oblique profile of configuration of a mountain glacier with a cross section at its edge.
Figure 2.6B. Cross section of mountain glacier showing uppermost zone of accumulation and lowermost zone of ablation. Névé or firn is recrystallized snow flakes; the firn line or equilibrium line is the place on the glacier every year in balance between the uppermost zone of accumulation of the new snow, and the lower zone of wastage where the ice melts away.
Such glacier ice is long-term storage of potential water for use in later years. Because of climate change in the past few decades, however, glacier ice in Afghanistan has been melting away. Once it is gone, it will not come back for a very long time – maybe never. Some glaciers at lower altitudes in Tajikistan have been melting away too. Some glacier ice in Pakistan has been melting away, but in the highest Karakoram Himalaya and Nanga Parbat Himalaya, some glaciers have been growing because of global warming producing greater evaporation from the sea which produces increased snow at high elevations.
All glaciers depend on snow for accumulation to eventual ice and they flow downhill to warmer locations where they melt away in what is accounted for by their mass balance (measures of accumulation against wastage). This melting wastage is controlled mainly by summer temperatures, which themselves are controlled by the amount sunlight that falls on the ice to melt it, and the cloudiness that occurs to reduce the sunlight.
E. Snow and ice melt becomes river water
The melt waters of snow and ice flow downhill to become river water. Because glacier ice in the mountains grinds up the rock that it moves over, a huge amount of sediment from glaciers gets into the river water where it is carried downstream. This sediment fills up behind dams and reduces water storage and water use behind the reservoir dam so the sediment must be removed if possible. Several engineering techniques have been developed to stop sedimentation from going in behind dams, or to remove excess sediment in reservoirs but these techniques have not been much used, if at all in Afghanistan and Pakistan.
F. Rain and snowmelt on hillsides
When it rains on hillslopes, the splash of the raindrops makes the small soil particles move around (Figures 2.7A & B).
Figures 2.7A & B. Illustration of rainsplash erosion and soil-particle mobilization, transport,and deposition.
Some water soaks (infiltrates) into ground and adds to essential ground-water storage that can be accessed by karez and tube wells. Some water runs off on the hillsides, which causes erosion of soil in which plants grow. This soil erosion is a most serious hazard in Afghanistan, Tajikistan, and Pakistan and should be protected against by construction of hillside benches and terraces, and with small rock check-dams in the low places where the water runs (Figure 2.8). These check-dams trap the soil erosion and let the runoff water soak into the ground to save it for use later as ground water.
Figure 2.8. Small check dams in Afghanistan and Pakistan built of rock rubble by local people to help slow down and control water runoff and soil erosion. Photograph of gully in Afghanistan where a small check dam of rocks has been installed to help prevent further soil-erosion transport downhill. Many such check dams and slope terraces were built with money from foreign military agribusiness development teams (ADTs) but because the indigenous knowledge of reasons to do this is general lacking amongst the tribesmen, the prolongation or maintenance of such intelligent actions into the future is not likely to occur.
G. Runoff water flow
Water that runs off the top of hill slopes first forms thin sheets of water that cover nearly all the ground in what is termed, sheet flow or overland flow (Figure 2.9). Some little way down the hill slope, however, the thin water sheets begin to collect more in slightly deeper depressions of a few centimeters that are known as rills to produce rill flow. The rills join together further downslope and produce deeper gulley flow, which themselves join together further downhill to produce stream flow or river flow.
Figure 2.9. Drawing of a hillslope showing upper area of sheet flow and sheet erosion, the midslope region of rill erosion, and the lowermost. where the runoff water is concentrated to produce gulley erosion.
H. Soil erosion
The splash of raindrops on the soil picks up little soil particles and moves them in the splash of the water. At the top of the hillslope, the overland sheet flow produces sheet erosion. Lower down the slope, the rill flow produces rill erosion, and where the rills flow together, gully erosion occurs (Figure 2.7A&B).
I. Drainage systems
Every stream, or segment of a river, is surrounded by its drainage basin, which is the total area of the land surface that contributes water into the stream (Figure 2.10).
Figure 2.10. Drawing of drainage basins separated by drainage divide between the basins.
These drainage basins are also referred to as watersheds, or the area which sheds its water. The high-point line or region that separates one drainage basin from another is a drainage divide. Drainage basins range in size from less than a square kilometer, to vast areas of subcontinental dimension. For example, the huge Indus River drainage basin starts near Mt. Kailas in the Himalaya of Tibet in China, before flowing across the Himalaya Mountains of northwestern India and into the Karakoram Himalaya of northern Pakistan, past the Nanga Parbat Himalaya, and out onto the lowland plains and plateaus at the Tarbella Dam site at the front of the mountains. In Afghanistan, the Kabul River and the Panshir River flow together at the Sarobi confluence, before going on in the Jalalabad Basin to receive the Kunar River tributary and thereafter flowing across the border with Pakistan to receive the Swat River tributary before joining the Indus River at Attock (Figure 2.11). The drainage basin of the Kabul River thus includes most of eastern Afghanistan and part of Khyber Pakhtunkhwa Province in Pakistan.
Figure 2.11. Maps of the drainages of the (A) Indus River and the (B) Kabul River tributary to the Indus.
J. Climate controls
The natural water supplies of any country are generally that which comes from the sky, either on an annual basis to produce river runoff, or the portion that is stored underground, perhaps derived from precipitation that has been received and held underground for a very long time. Thus in order to understand the watersheds and their importance in arid countries such as Afghanistan, Tajikistan, and Pakistan, we must first consider the climate of the region, with its variations in precipitation, as we focus on the all-important hydrology of the Hindu Kush, Pamir, and Western Himalaya, mainly in terms of the effects of glacier ice, the rivers, and the lakes.
The climates of the Earth are controlled largely by the amount of precipitation and temperature delivered at different times and places as a function of latitude, altitude, and position on a continent relative to ocean water masses. Mountainous and desertic countries such as Afghanistan, Tajikistan, and Pakistan, the interiors of which are far from oceanic moisture sources and subject to the vicissitudes of variable winds and erratic supplies of precipitation, are characterized also by extremes of what is called continentally wherein winters can be quite cold and summers excessively hot and dry so that any water derived from snow melt and rainstorm precipitation is essential but evaporates quickly.
In general, Afghanistan, Tajikistan and Pakistan are dominantly arid to semi-arid regions, except in the frontal or foothill mountains where more orographic precipitation is caused when air masses are forced to rise and the attendant cooling causes condensation of moisture out of the air (Figure 2.4). The amount of precipitation increases to the northeast in Afghanistan (Figure 2.5), and to the north in Pakistan, in response to the higher altitudes there. Average annual precipitation is commonly <210 mm in many areas, declining to <110 mm in the southwestern deserts, and increasing to >1000 mm in the high mountains. In all, over 80 percent of the Afghanistan’s water resources have their origin in the mountains >2000 m in altitude, which function as a natural storage of snow and ice that supports perennial flow in all the major rivers in summer. Such is also true of much of Tajikistan and Pakistan, although percentages and altitudes vary.
In Afghanistan, the main period of precipitation is borne on the westerly winds and extends from November to May, but is shortened in the south to December through April. About 50 percent of the precipitation occurs in winter (January to March), mostly as snow. A further 30 percent falls in spring (April to June). In the summer season some monsoonal precipitation makes it into extreme southeastern Afghanistan along the border with Pakistan, although occasionally monsoon moisture sources go well into the Hindu Kush in Central and Northern Afghanistan.
The water cycle is absolutely critical to modern Afghanistan, Tajikistan, and Pakistan, just as it has been nearly totally responsible for so much of the geomorphic landforms and a large share of the natural hazards of the three countries. As overall semi-arid to arid nations, a number of places in the north and south have no surficial drainage. Elsewhere, however, the drainage net is well developed and provides the basic water source to the people, even as it is also responsible for the general character of most of the landscapes (Figure 2.12).
Figure 2.12. River basins of Afghanistan.
The watershed boundaries in Afghanistan begin high on ridge and mountain tops, generally but not entirely, in the middle and northeast of the country and thereafter the rivers flow radially outward over the borders into neighboring countries. This position at the top of the watersheds places Afghanistan geomorphically as a source of snow and glacier-ice storage and melt-water river delivery downstream that combine to produce most of the geomorphology or landforms of the country. Below the land surface of the nation the fractures in the bedrock, and the natural porosity in the sediment-filled basins throughout the nation (Kabul, Jalalabad, Seistan, northern basins), have considerable storage of underground water that is obtained through gently inclined karez tunnels, as well as boreholes and pumps.
Similarly in Pakistan the high mountains in the north receive plentiful precipitation, particularly from the winter Westerlies, but also to a certain extent in the foothills to the Himalaya and to the south, a certain amount of monsoonal precipitation occurs. In general in Pakistan then, the precipitation received in the country can be divided into two main seasons: the summer monsoon that comes in from the east and northeast from July to September, and the winter westerly disturbances that come into the country from Afghanistan and Iran from December to March. In spite of severe flooding in 2010 and 2011, recent analyses show a significantly decreasing trend of precipitation all over the country, with periodic prolonged droughts that will pose severe risks to agriculture and the water-management sectors in Pakistan.
In Tajikistan the climate is continental, subtropical, and semiarid, with some desert areas. The climate changes according to elevation. In the subtropical southwestern lowlands, which have the highest average temperatures, the climate is arid. The average annual precipitation for most of the republic ranges between 700 and 1,600 millimeters. Most precipitation occurs in the winter and spring.