EQ1: What are the processes operating within the hydrological cycle from global to local scale? Flashcards

Question

What is the various terminology associated with a water budget diagram?

Answer 1

WATER BUDGET 1. Soil Moisture Surplus - Precipitation is greater than potential evapotranspiration and the soil water store is full, so there is a surplus/excess of soil moisture for plant use, runoff into streams and recharging groundwater supplies. 2. Soil Moisture Utilisation - Potential evapotranspiration increases and exceeds precipitation, so there is more water evaporating from the ground surface and being transpired by plants than is falling as rain. Water is also drawn up from the soil by capillary action. The water is gradually used up faster than it is replenished. 3. Maximum Annual Temperatures - High temperatures cause maximum evapotranspiration, precipitation is at a minimum and therefore plants use up the soil moisture store. River levels will fall and crops will need irrigation. 4. Soil Moisture Deficit/Defiancy - The soil water store has been used up by high rates of evapotranspiration and low precipitation. Rivers run dry and drought ensues. Plants can only survive if they are adapted to periods of drought or are irrigated. 5. Soil Moisture Recharge - Occurs when moisture from precipitation infiltrates and percolates in the stores to repay the soil moisture utilisation/deficit. 6. Field Capacity - The normal amount of water that can be held in the soil. At this point, the soil is full of water and cannot hold any more. 7. Potential Evapotranspiration - An estimate of the amount of water that could be lost by evapotranspiration, in a given period, if there was sufficient moisture available (depending on temperature and air humidity). 8. Actual Evapotranspiration - The amount of water that is lost through transpiration (release from leaves) and evaporation (heating of water on surfaces) to the atmosphere.

Answer 2

WATER BUDGETS AFFECTED BY CLIMATE 1. Water Budgets vary with climate type and acts as a useful indication of available water supplies. a) Places of higher latitude being more familiar with precipitation in the form of snow or sleet, which is stored in the colder months (Winter/Spring) due to the colder temperatures. This store is then released as meltwater under a warmer climate (Summer). b) Temperate locations will see more consistent precipitation, yet temperatures are seasonal, with summer seeing soil moisture utilisation and deficit. c) Places closer to the equator will expect warmer climates throughout the year as well as lower rates of precipitation (other than the rainforested areas), this means that for the majority of the year there is soil moisture deficit. - Flash flooding is common in drylands, where sun-baked soils cannot absorb intense storm rains fast enough. The surface soil becomes saturated, causing rapid surface runoff. 2. Barrow, Alaska (Polar Example) 3. Cairo, Eygpt (Tropical Example)

Answer 3

RIVER REGIMES - Indicates the annual variation in the discharge of a river at a particular point/gauging station and is measured in cumecs. Much of this river flow is not from immediate precipitation or run-off but supplied from groundwater between periods of rain, which feeds steadily into the river system from base water flow. This is influenced by climate, geology, soils and human activity (land-use changes). 1. Simple Regime a) Rivers experience a period of seasonally high discharge, followed by a low discharge. b) Often rivers when inputs depend on glacial meltwater, snowmelt or seasonally storms. c) Rivers within temperate climates, which rise in mountainous regions where summer snowmelt occurs tends to be like this. 2. Complex Regime a) Larger rivers cross several different relief and climate zones and therefore experience the effects of different seasonal climatic events. b) True of rivers such as the Mississippi (covers 40% of the continental United States) or the Ganges influenced by numerous tributaries from different altitudes/climates. c) Influences diminish extreme discharges and increase the regularity of the mean monthly discharge from upstream to downstream. d) Human factors also contribute to their complexity, such as damming a river for energy or irrigation.

Answer 4

RIVER REGIME INFLUENCES 1. Climate Zones a) The longer the river, the more complex its variables tend to be. b) As the river may cross several climatic zones and encounter very different land uses and population densities along their length. c) Mountain climate zones have a high river flow in Spring caused by melting snow and ice. d) Tropical/Mediterranean climate zones have distinct wet and dry seasons, desert zones have dried river systems. 2. Size of the River a) Large rivers can have complex regimes from varied catchments. 3. Precipitation a) The amount, type, pattern and intensity. b) Regimes often reflect rainfall seasonal maxima or when snowfields or glaciers melt (Late spring/ Early Summer). 4. Temperature a) Evaporation is marked in summer with higher temperatures. b) Climate change will mold annual river regimes. c) River regimes are annual so large fluctuations will be due to seasonal change. 5. Geology and overlying soils a) Permeability and porosity as water can be stored as groundwater in permeable rocks and is released to the river through base flow with a regulated flow during dry periods. 6. Amount and type of vegetation cover a) Wetlands can hold water and release it very slowly into the system. 7. Human influences a) Dams building can regulate the flow.

Answer 5

RIVER NILE REGIME 1. Shaped by the interaction between climate and geography across the basin. a) Drainage basin cover 10% of Africa and stretches across 35 degrees latitude, crosses climate zones ranging from high rainfall of tropical grasslands (extensive wetlands) to arid desert (barren desert) conditions. b) Hydrological characteristics of the River Nile at any location are affected by upstream geographical and climatic conditions. 2. The water of the River Nile originates from two major sources both within humid regions, rainfall averages 1,000 mm per year. Only covering a mere 20% of the basin. a) The mean water discharge is 2,800m^3/s. - The Equatorial Lakes Plateau, White Nile (30%). - Highlands of Ethiopia and Eritrea, Blue Nile (56%) and Atbara (14%). b) Yet in some parts of Egypt rainfall drops to less than 20 mm per year. - Nearly 50% of the river Nile flows through areas with low to no rainfall. - Discharge of the Blue Nile and Atbara exhibits high seasonal variation due to the seasonal rains over the Ethiopian highlands, which ultimately leads to annual Nile flooding. - And most precipitation falling in the Nile basin is lost to evaporation, seepage and overflow into wetlands each year. - So discharge of the River Nile is low compared to other large rivers even though it is the world’s longest river

Answer 6

STORM HYDROGRAPH - Shows variation in a river’s discharge at a given point over a short period of time (usually before, during and after a storm). - The shape of the storm hydrograph changes as a result of physical and human factors and can be described as either flashy or subdued. - Can show how a river responds to a particular storm and is useful for predicting flood risk and comparing drainage basins responses to a heavy precipitation event. - Climate change is likely to change hydrographs by making precipitation more intense in the Northern Hemisphere and drier in tropical areas. 1. Flashy Hydrograph - Indicate that there is a rapid increase in discharge, and perhaps a high risk of a sudden flood. 2. Subdued/Attenuated Hydrograph - Indicates that there is a limited increase in discharge following a weather event.

Answer 7

STORM HYDROGRAPH TERMINOLOGY 1. Normal (Base) Flow - The normal, day to day discharge of a river. 2. Storm Runoff - The part of the river flow derived from the immediate rainfall event. The most rapid transfer of water occurs overland and via throughflow. 3. Rising Limb - The increase in discharge in response to surface runoff and throughflow from a rainfall event, until peak flow is reached. When the storm begins, the river’s initial response is negligible as the precipitation takes time to reach the channel. 4. Falling (Recessional) Limb - The decline in discharge that occurs after peak flow. This segment is usually less steep than the rising limb because throughflow is being released relatively slowly into the channel. 5. Lag Time - The difference in hours and minutes between the time of maximum precipitation and the time of peak discharge. The lag time varies according to drainage basin conditions. Rivers with a short lag time tend to experience a higher peak discharge and they are more prone to flooding than rivers with a long lag time. 6. Approach Segment - The discharge of the river before the storm (the antecedent flow rate). 7. Peak Discharge - The maximum discharge by a stream or river in response to a rainfall event. 8. Bankfull Discharge - Occurs when a river’s water level reaches the top of its channel. Any further increase will result in flooding of the surrounding land.

Answer 8

STORM HYDROGRAPH FACTORS 1. Drainage Basin Size. a) Small basin size: water will reach the channel rapidly. b) Large basin size: water has a further distance to travel, longer for peak flow to occur. 2. Drainage Basin Shape. a) Circular basins: it will take less time for the water to reach the channel as all the extremities are equidistant from the channel. b) Elongated basins: it will take longer for the extremities to reach the channel. 3. Drainage Basin Relief a) Steep slopes: water flows rapidly downhill, reaching the channel faster. b) Gentle slopes/flat terrain: water can infiltrate into the ground and travel slowly through soil and rock. 4. Soil Type a) Low infiltration: clay and thin soils have low porosity and the grains swell when they absorb water, so water infiltrates slowly. And thin soil becomes saturated quickly. b) High infiltration: sandy and thick soils have a higher porosity, so the water can infiltrate. Deep soils allow more infiltration. 5. Geology Type a) Impermeable rocks: (granite) water cannot percolate into the rock, increasing surface runoff to the rivers. b) Permeable rocks: (limestone) water percolates through pore spaces and fissures into the groundwater store. 6. Drainage Density a) High drainage density: a large number of surface streams per km² means the stormwater will reach the channel rapidly. b) Low drainage density: a small number of surface streams per km² means the water travels slowly through the soil and rock to the river. 7. Natural Vegetation a) A low density/sparse vegetation: such as thin grass means low levels of interception and rapid movement of water. b) Densely forested woodland: deciduous in summer means more interception with more water lost to evapotranspiration from the surface of vegetation and slower movement of water. 8. Land Use a) Heavily urban and deforested: Concrete surfaces and roads have a lower permeability, and drains often carry water directly to the river. Ploughing furrows up/downslope increases runoff. b) Rural and reforested: Vegetated surfaces intercept water and allow infiltration so water travels slowly to the river channel. 9. Water Management a) No Intervention: Unregulated flow means shorter lag time as rainfall will reach the river faster. b) Intervention: Dams regulates flows downstream and abstraction lowers groundwater levels, increasing percolation. 10. Precipitation Intensity a) High Intensity: when precipitation exceeds the infiltration capacity of the soil, surface runoff occurs and transports river rapidly to the channel. b) Low Intensity: steady precipitation can infiltrate into the soil and then travel slowly through the soil to the river channel. 11. Precipitation Duration a) Prolonged: the water table rises and the soils become saturated, causing surface runoff. b) Short duration: most of the water infiltrates into the soil and travels at a slower pace. 12. Snowfall a) Rapid snowmelt: meltwater can not infiltrate into the frozen ground, so it flows rapidly over the surface into the channel. b) Gradual snowmelt: the ground thaws with the snow, so the meltwater can infiltrate into the soil and rocks before reaching the channel. 13. Evapotranspiration a) Lower rates/temperatures: fewer losses from the drainage basin system will increase the discharge. b) Higher rates/temperatures: high evapotranspiration losses will reduce discharge into the river channel.

Answer 9

PLANNERS IN LAND USE 1. UK planners are required to determine whether any proposed development will influence flood risk with the change in land use, the National Planning Policy Framework (NPPF) sets out strict guidelines. a) Decisions vary depending on how much weight is given to environmental factors versus economic development (as it’ll increase the likelihood of higher flow and faster response time). 2. Building activity leads to the clearing of vegetation, which exposes soil and increases overland flow. a) Drains and sewers are built, which reduce the distance that stormwater must travel before reaching the channel. b) Urban rivers tend to be channelised with embankments to guard against flooding, yet can often be more devastating as the river overtops defences in a very confined space. c) Bridges can restrain the free discharge of floodwaters and act as local dams for upstream floods. 3. SuDS (Sustainable Drainage Systems) have been introduced to reduce runoff produced from rainfall such as: a) Green roofs - vegetation planted on a waterproof membrane to absorb and store rainfall. b) Permeable pavements - infiltrate rainwater. c) Filter drains - trenches filled with gravel, runoff storage and delay runoff time. d) Detention basins - depression in the land which delays storm runoff.

EQ1: What are the processes operating within the hydrological cycle from global to local scale? Flashcards

(33 cards)