The Hydrologic Cycle

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Geographical Biogeosciences
GEOB 103
Brett Eaton

GEOB103 The Hydrologic Cycle - The scientific study of the distribution and properties of water within the atmosphere and at the Earth’s surface o Precipitation, evaporation, evapotranspiration, runoff, soil moisture, & groundwater Water facts: - Water covers 70% of the Earth’s surface - Canada has about 9% of the world’s renewable freshwater supply - The human body is about 70% water - Nearly all industrial processes require water - Nearly all metabolic (i.e. life) processes require water - 80 countries worldwide currently face water shortages THE OCEAN - Largest water storage sites – holding 97% of the world’s water - Most evaporation occurs over the world’s oceans - 86% of the water vapor in the atmosphere can be traced to an ocean source - Most precipitation occurs over the world’s oceans THE ATMOSPHERE - All the water vapor is held here - Source of all precipitation - It is the process of evaporation followed by condensation that purifies water THE LAND SURFACE - While most hydrological cycles bypasses the land surface completely, this is where we live and where geomorphology happens. Components of the hydrological cycle: - Evaporation combined with plant transpiration contributes 14% of the atmospheric water vapor - Precipitation over land surfaces is only about 22% of the total worldwide precipitation amount. 20% comes from water that evaporated from oceans and 22% comes from the water that evapotranspired from the continents - Water falling on a land mass will either (1) return directly to the atmosphere via evapotranspiration (14% of the 22% precipitation); or (2) find its way to the ocean through the stream channel network or via the groundwater system (8% of the 22% precipitation) - Water may also be stored on the land surface, as well as in the atmosphere and the oceans. The time a water molecule spends in any particular storage location (e.g. in a river, in groundwater, in a glacier, in the atmosphere) is referred to as residence time STORAGE LOCATIONS & TYPICAL RESIDENCE TIMES 3 3 - The units for the volumes of water stored are 1000s of cubic kilometers, written x 10 km . - The total volume of water is about 1,400,000 x 10 km , most of which is stored in the ocean 3 3 (1,365,000 x 10 km and is salt water 3 3 3 3 3 Storage Locations (1000 km ), 1 x 10 km , V(10 km ) T R Biosphere ~1 7 days Atmosphere ~10 10 days River systems ~2 14 days Swamps, Lakes, Reservoirs ~100 3500 days Soil water ~70 1-350 days (dependent on soil type) Groundwater ~8000 35 million days Icecaps and Glaciers ~27,000 ~1000 years * Icecap climate records > 400,000 years THE WATER BUDGET OR WATER BALANCE In order to understand where water is stored and how it moves, it helps to make use of the fact that the total volume of water on the Earth is constant. This means that we can set up a budget to track water: Precipitation = Evapotranspiration + Runoff + Changes in Storage P = ET + R + ∆S All quantities are measured in water depths (mm). The basic equation can be rearranged any way you like, depending on what the question you are interested in. The components can also be expanded. Interested in predicting runoff from a stream? R = P - ET - ∆S Interested in where water is stored in a watershed? ∆S + iS + ∆L = P G R - ET - Where ∆S is the storage as ice, ∆S is storage in lakes/rivers, and ∆S storage in groundwater. i L G PRECIPITATION - From the point of the terrestrial hydrologic cycle, the process begins when it rains, shows, or otherwise sends water downward to the earth. Precipitation is the input to the cycle. - Precipitation types: we generally recognize 2 general types of precipitation o Rain: condensed water vapor in drops large enough to fall under the influence of gravity o Snow: solid precipitation consisting of flakes that gets smaller as the temp. gets colder SPATIAL AND SEASONAL VARIABILITY IN PRECIPITATION Spatial distribution of precipitation determines the relative importance of the other components of the hydrological system (i.e. the balance between evaporation, transpiration surface runoff and groundwater runoff) Spatial Variations: Precipitation amounts very with: 1. Location: precipitation is always greater near the source of moisture (almost exclusively the oceans). So precipitation rates are high near the coast and decrease as air masses move inland over the continent 2. Elevation: as air masses are forced upwards, they are cooled and their ability to hold water is reduced. The result is more rain at higher elevations, particularly where air masses blow off the ocean and up the side of a mountain rain (like in Vancouver). This is called the orographic effect, and the rain or snow produced this way is called orographic precipitation 3. Lakes: lakes act like small sources of water vapor (like oceans), producing local increases in precipitation for the upwind land areas SEASONAL VARIATION - Ex. Areas that are subject to monsoon rains experience extreme variations in precipitation inputs from nearly desert conditions to rainforest conditions, all within a single year. The switch from one type to another is often predictable to within about one week. However, climate change may be disrupting the processes of brining the rainy season. - Summer precipitation patters in the lower mainland: locally, we receive far less precipitation during the summer months. Small frontal storms or convective precipitation bring (usually) short rain storms with only a localized spatial extend. This is because a high pressure cell typically develops in the atmosphere that directs cyclonic storms (frontal systems) further north - Winter precipitation patters, Vancouver: in the winder, cyclonic storms (frontal systems) track one after another. These systems produce long duration, spatially extensive precipitation for at least several hours, usually. INTERCEPTION - Once rain reaches the Earth’s surface, some of it will make its way into the soil or flow over the soil. But some of it will be trapped above the ground my vegetation - The dominant component of all kinds of interception is the canopy interception component - The amount of interception that occurs during a storm depends on: how much interception capacity that canopy has; how much water is already stored in the canopy, what the weather conditions are like 1. Previous weather: generally, if the canopy is initially dry, much more rain will be intercepted. 2. Storm duration: as a storm goes on, the canopy begins to become “saturated” and the rate of interception declines 3. Storm intensity: a high intensity rainfall will have larger, faster falling rain drops that are less likely to be intercepted 4. Wind speed: high winds can dislodge the intercepted water, causing throughfall Interception Rate = P(B) – P(A)/T of measurementow long left out) EVAPOTRANSPIRATION - The loss of moisture from the Earth’s surface by way of evaporation combined with plant transpiration. The process can only occur if there is an input of energy (i.e. the Sun) - There are 2 kinds of evapotranspiration: o Potential Evapotranspiration (PE): the amount of evapotranspiration that would occur if the water supply were unlimited. It represents a theoretical maximum loss. The rate of potential evaporation is controlled by: 1. Energy availability at the surface (temperature, sunlight, etc.) Energy can come directly from the sun (as visible light) Energy can come indirectly from the atmosphere (heat, ultraviolet radiation) The available energy is highest near the equator and decreases toward the poles and varies seasonally 2. The ease which water vapor can be diffused into the atmosphere (wind & turbulence) Actual evapotranspiration is affected by: 1. The depth below the surface of the groundwater 2. The size and density of soil voids (which is called its porosity), because this relates to house easily water and air can move through the soil to the surface 3. Humidity of the air mass above the soil (how dry is the air) 4. Vegetation type & growth stage 5. Soil type and soil water storage capacity (related to #2) but this is about moisture available to plant roots. Note: We find arid areas near the equator, where this is abundant available energy for evapotranspiration, but very little available water (so PE is always much greater than actual ET). Humid regions are found north of the arid zones where this is less available energy for PE and more abundant moisture supply. Here, PE is closer to (but still usually greater than) the actual ET. Runoff Processes Water that condenses to form precipitation that falls upon the land surface and is not intercepted and evaporated or transpired will either infiltrate the soil, possibly reaching the deeper groundwater reservoir, or it will runoff over the ground surface. If the water infiltrates, it is referred to as soil moisture. Soil moisture: the volume of water in the unsaturated zone immediately below ground surface. It is that part above the soil above the groundwater table. The movement of water into the soil surface (or the Vadose Zone) is referred to as infiltration. We can make a distinction between the infiltration capacity and the infiltration rate. - Infiltration Capacity is the maximum rate at which water can move into a soil under a given soil moisture condition. Infiltration capacity changes with the amount of moisture already in the soil. It only corresponds to the actual rate at which water enters the soil when water availability is not a limiting factor. o When a soil is dry, it acts as a sponge that sucks moisture down into the soil (capillary tension). This is influenced by gravity o When a soil becomes saturated, the capillary tension is reduced, and the maximum rate of infiltration is determined by how fast water can move downwards through the soil. This has the lowest possible infiltration capacity. - Infiltration Rate is the rate at which water actually penetrates the surface of the soil. The infiltration rate is limited by the infiltration capacity and the rate at which water reaches the soil surface (i.e. the rate of precipitation or, possibly, the rate of snowmelt) - Both infiltration capacity and infiltration rate are normally expressed in units of cm/hr or mm/hr, because this is the way precipitation is expressed. Percolation: the vertical and lateral movement of water through the soil by gravity Porosity: the percentage of the volume of the rock that is open space (pore space). Soil moisture is often expressed as the ratio of water volume to pore space volume and varies from 0% (dry soil) to 100% (fully saturated soil) FACTORS THAT CONTROL INFILTRATION RATE 1. Precipitation characteristics: type, intensity, duration affect the amount of precipitation reaching the surface and the soil moisture levels 2. Surface topography: roughness & slope determine the likelihood of surface runoff (hills vs roads) 3. Vegetative cover: determines interception, prevents soil compaction; rood development provides pathways for percolation into the soil 4. Soil conditions a. Soil texture (e.g. sand will have a high infiltration rate, clay will have a low infiltration rate) b. Porosity (is the soil highly porous with well-connected pores or not?) c. Compaction (this will reduce the space between the pores & surface – compaction can prevent water from easily entering an otherwise porous soil, such as the compaction of a forest soil) d. Structure & layering (layers or structures can act as preferential flow paths for percolation) e. Moisture content (this affects the infiltration capacity) GROUNDWATER - The part of the subsurface water below the water table, where the pores are fully saturated. Groundwater may be found in bedrock or soil, depending on the position of the water table (which varies seasonally). - The total estimated groundwater volume is enough to cover all land surfaces to a depth of 60,000 mm or 60 meters. It is a large water storage zone - Water flows through the groundwater system very slowly (less that 1m/hr is typical, and 5 m/hr would be a fairly high rate). It contrast, streams often flow at rates of 1-5 m/second - Is available, at least in small amounts, nearly everywhere, though the quantity varies Why is it important? - Water supply - Irrigation/farming o Agricultural irrigation is the largest use of groundwater - Mining oil and gas - Maintenance of stream-flow and the groundwater-dependent ecosystems RELATED TERMS: - Aquifer: a layer of rock or soil from which water can easily move o All aquifers have high hydraulic conductivity(water can flow easily through them) o These will be the highly porous soils and rocks that have relatively large pores and therefore weak capillary forces. o Almost all aquifers are sedimentary deposits or sedimentary rocks (i.e. sand(stone)) o TYPES OF AQUIFERS:  Unconfined aquifer  Shallower  No confining layer  Local recharge  Has water table  Susceptible to contamination  Confined aquifer  Deeper  Confining layer  Distant recharge  Water under pressure  Less susceptible to contamination  Artesian well  Water in confined aquifers are under pressure – a crack in the rock can cause the water to shoot up - Aquitard: A layer of rock or soil that only marginally allows groundwater movement o Tends to be rocks of low porosity and relatively small pores, with a low hydraulic conductivity o Most igneous and metamorphic rocks are aquitards (i.e. granite, limestone, shale) - Aquiclude: a layer of rock or soil that does not allow the movement of groundwater o Rocks and soils rich in clay: when wet, minerals absorb water & swell, causing all pores to be blocked off and preventing any measurable groundwater flow from occurring Material Type Typical Porosity Status Limestone 3 – 30% Aquitard Soils 50 – 65% Aquifer Shale 3 – 12% Aquiclude Silt 35 – 50% Aquifer Igneous Intrusive 2 – 10% Aquitard Sand & Gravel 20 – 40% Aquifer Aquifer Sandstone 10 – 25% fer Recharge and Discharge areas - Recharge area o Where the groundwater is replenished o An area where precipitation is transmitted downwards to an aquifer - Discharge area o Where groundwater leaves the aquifer o i.e. springs, seeps, rivers, lakes Seasonal Patterns - Patterns of recharge and discharge change seasonally (similar to variable sourc
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