366 Water in Deserts

Geo Factsheet www.curriculum-press.co.uk Number 366 Water in Deserts Figure 1 The global distribution of deserts ● The sparsity of vegetation cover means that there is The action of flowing water, both in the present and past, is, and has an absence of plant roots. Humus in the soils is limited; been, important in shaping desert landscapes. Rainfall quantities in existing soils are compact and infiltration is limited. deserts are low overall but rainfall events that do occur can have a ● Interception is minimal. The rainfall hits the ground marked influence on the landscape. Many landforms in deserts are and dislodges fine, loose particles, moving them by rain shaped by the action of rivers and the water that flows into and / or splash. They can resettle and become lodged in any pore through the region. Figure 1 shows the global distribution of deserts. spaces that did exist in the soil, acting to ‘plug’ the gaps and create a top soil layer which has reduced permeability. Rainfall in deserts Such soils can only allow rainwater to infiltrate at a rate Desert regions can be classed as hyper-arid, arid or semi-arid of around a few mm per hour, so any excess rainfall will depending on the average annual precipitation that they receive: build up on, and flow over, the ground surface. Hyper-arid zones have a mean annual precipitation value of less than 100mm; Arid zones have an annual rainfall total of less than 250 mm; Semi-arid areas have an average annual precipitation of 250-500 mm. Runoff in deserts Rainfall in deserts can have a significant impact on the landscape in a relatively short period of time because large amounts of runoff are generated in desert environments after episodes of rainfall. This is due to: ● Low levels of vegetation cover, which means that absorption of the rainfall by vegetation is limited and the ground saturates more rapidly, reducing the potential for infiltration and increasing overland flow. Figure 2 How the hydrological cycle operates in deserts 1 Water in Deserts Geo Factsheet 366 Much of the episodic surging river discharge experienced in hot Rivers and drainage in hot deserts deserts is therefore derived from infiltration-excess overland flow, Desert rivers can be considered in different ways: which is when precipitation exceeds the infiltration capacity of the ● Perennial rivers have a continuous flow all year round land and so flows over the surface. Overland flow may spread out and in years providing that what is considered to be ‘normal’ travel across the desert surface as a sheet flood as it moves towards (or in excess of normal) rainfall occurs for a given place. river channels, or may be carried by relatively small ‘feeder’ channels ● Perennial rivers that flow continuously are uncommon called rills and gullies. Desert rivers, when in flow, carry out processes in arid zones due to the unreliable and infrequent nature which play a significant part in shaping the desert landscape;eroding , of precipitation. Therefore any perennial desert rivers transporting and depositing material. tend to be exogenous, meaning that the river originates from outside the desert. Examples of large perennial Fluvial Erosion rivers are the Blue Nile (a major tributary of the Nile) A flowing desert river may erode its channel through hydraulic which originates in the Ethiopian Highlands outside of action when the pressure and sheer force of the flowing water the Sahara Desert region through which the Nile flows, contacting the bed and banks of the river channel removes and the Colorado River which originates in the Rocky particles; abrasion when the large load carried by the river rubs Mountains outside of the Great Basin, Sonoran and and scrapes away material from the bed and banks, and solution Mojave Deserts through which the river flows in the (corrosion) when chemical processes dissolve the rock and remove American southwest. small particles. ● Rivers in the arid tropics are more likely to be ephemeral which means that they only flow occasionally (during and Fluvial Transport following episodes of relatively heavy rainfall). The flow Due to the lack of vegetation in hot, arid environments, there is of such rivers is unpredictable, irregular and may not last little to consolidate the ground, meaning that a large amount of for long, but when rivers do flow, they exhibit high levels loose weathered material is available. The fast-flowing, ephemeral of discharge, meaning they possess a high level of erosive rivers therefore pick up and transport much of this loose material – power and the ability to entrain and transport large loads. desert rivers are capable of carrying very large loads. The river’s ● Perennial or ephemeral rivers in the arid tropics may be load is transported either along the bed by traction or saltation, is endorheic, meaning that they have a centripetal inward moved as a suspended load or dissolved in solution. flowing pattern of drainage, with rivers flowing towards large inland lakes, basins, or salt pans. Examples of such Fluvial Deposition drainage patterns include those where channels flow into The ephemeral nature of many desert rivers means that they have Lake Chad and Chott el Djerid (Tunisia); both in the a discontinuous and fragmented flow. The water that feeds the Sahara region and the Great Salt Lake in Utah, USA (see rivers comes in short bursts during storm events and therefore river section on playas). flow will increase and cease periodically. Velocities will fall as ● In the great sand seas or ergs such as the Grand Erg discharge levels decline after storm events, with water being lost Occidental, which is located in a hyper-arid zone in from the channel through evaporation, infiltration, and percolation. Algeria (Sahara), drainage patterns are absent, meaning This means that the river loses the ability to transport its load and that surface impressions left by flowing rivers do not exist. will deposit it readily. Water will flow in these regions, albeit rarely, but barely leaves a mark due to the high porosity of the sandy material, through which water infiltrates, and is ‘lost’ underground. Figure 3 Landforms created by fluvial erosion & deposition in deserts 2 Water in Deserts Geo Factsheet 366 Fluvial landforms in hot deserts 2. Wadis Although there is relatively little water in hot deserts, many of the Wadis (also known as arroyos), such as Wadi Rum which is situated erosional, and some of the depositional, landforms found there have in Jordan, and Wadi Bani Khalid in Oman, both within the Arabian been formed by the action of flowing water. Flowing water in deserts Desert, are dry river valleys which experience infrequent flows of can act as an agent of erosion, transportation, and deposition of water. They are deep, steep-sided ravines which vary in size from material and the landforms that these processes create are either being just a few metres to several hundreds of kilometres long. Wadis erosional or depositional in nature. The erosional landforms created are dry for most of the time but are filled with water derived from by flowing water in hot deserts include rills and gullies (sometimes overland flow, which may arrive through rills and gullies, during creating badlands), wadis (also known as arroyos), canyons, mesas irregular and infrequent flash flood events. When floodwaters subside and buttes (which may be classed as inselbergs) and pediments; the due to evaporation and infiltration, thick layers of poorly sorted depositional features include alluvial fans, bahadas and playas. sediment are deposited, creating a flattened-out channel bed. The flow of water within the wadi often takes place in narrow, criss- Desert landforms formed by water erosion crossing braided channels as the water navigates the wadi-floor 1. Rills and gullies deposits. In semi-arid desert fringes, there is an average of one flood Some of the infiltration excess overland flow, a common occurrence a year, but in the more arid zones, wadis may go for ten years without due to the nature of the climate and land in the arid tropics, may form carrying water. Due to the paucity of and infrequency of water features called rills and gullies. These small channels, carved out entering the dry valley, it is suggested that wadis are relict features by the action of water running over the desert surface, can be found which could only have been formed at a time in which the climate of embroidering desert hillslopes. the place in which they are situated was wetter (see ‘pluvials’ below), Rills are steep-sided, narrow watercourses which carry water during with greater and more frequent rainfall events. times of high precipitation. Rills are small features which can be 5-30 3. Canyons cm deep and 22-100 cm wide. Rills may be just temporary features, Canyons, such as the Grand Canyon in the southwest of the USA (see and can be destroyed during dry periods. Rills enlarge through a Figure 5) are steep-sided, deeper versions of wadis. They are created coalescent process called cross-grading. They are considered to be by the incision (vertical/downward erosion) by the action of flowing an intermediate stage between water being transported as merely water in deserts. Their often quite significant depth cannot be created overland flow and the development of a more considerably-sized by intermittent incision by ephemeral desert rivers and therefore it gulley. may be that they have been created by a perennial exogenous river, Gullies are larger channels that are a more permanent feature on flowing into the arid region from elsewhere, or when the ephemeral the landscape than rills (yet the flow of water within them is still yet powerful water flow is combined with tectonic uplift of land.

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