Chapter 6
Evapotranspiration
(ET) Combines evaporation from the land surface and vaporization through the stomata in plant leaves -Difficult to measure how much water evaporated from the soil and how much from plants, so they are usually measured together as ET -Evaporation can be seen as waste water, but u gotta transpire for basic processes -Potential evapotranspiration (PET) rate tells us how fast water vapor WOULD be lost from a densely vegetated plant-soil system if soil water content were continuously maintained at an optimal level -----PET = 0.65 x pan evaporation (0.65 is correction factor for well watered, dense vegatation) (pan evaporation is amount of water evaporated from an open pan of water of standard design) ex Class A evaporation pan -----Determined by vapor pressure gradient between a wet soil, leaf, or body of water & the atmosphere ---------> this gradient is influenced by solar radiation, temperature, relative humidity, cloud cover, and wind ---PET values range from 1500 mm per year in hot, arid climates to less than 40 mm in very cold regions. Temperate winters may have PET less than 1 mm per day. By contrast, hot, dry wind will continually sweep away water vapor from a wet surface, creating a particularly high PET levels as much as 10-12 mm per day
Groundwater Resources
-20% of water in US comes from groundwater -Unconfined aquifers commonly provide water for rural -Aquicludes: aquifers confined between impermeable strata -Municipal, Industrial, Irrigation water comes from larger, deeper aquifers which are replenished by slow, mainly horizontal seepage from recharge areas where aquifers are exposed to the soil -Theoretically could be considered renewable, but they are often tapped harder than they are replenished --> this lowers the water table --> in coastal areas, overpumping leads to saltwater intrusion SHALLOW GROUNDWATER: groundwater near the surface can serve as water reservoir for soil, can feed plants from capillary rise, but it also brings dissolved slats to the surface which can lead to soil degradation in dry regions
Chemical Movement Through Macropores
-Chemicals are normally broken down in soil by microorganisms within a few weeks, but large macropores allow them to bypass this zone PREFERENTIAL/BYPASS FLOW: Leaching of chemicals is most serious if applied on soil surface, when they are washed from soil surface into large pores where they move quickly downward. --Reduced by incorporating chemicals into the upper few centimeters of soil INTENSITY OF RAIN/IRRIGATION: high intensity water events make water+its chemicals move downward rapidly through macropores+cracks, gentle rains minimize fast percolation of water +chemicals
Fate of Incoming Water
-Interception by plant foliage and returned to atmosphere by evaporation without ever reaching the soil, in forested areas can prevent 30-50% of precipitation from reaching the soil -Interception and sublimation of snow is important in coniferous forests -Water that hits the soil surface can either infiltrate or runoff ----->If rate of precip>rate of infiltration, ponding may result, and considerable runoff and erosion may take place -Water can drain away from root zone, but can rise back up in dry seasons by capillary water -Rainfall will be more readily absorbed if its gentle and over a few days -Snowfall in the early winter can protect soil from freezing -Open and stable soil structure with lots of macropore space will do a LOT to encourage infiltration. also plant cover.
Control of Surface Evaporation
-Over half of precip in semiarid/subhumid areas is returned to atmosphere by E from soil surface; plant residues are sparse -E loss rob plant community of much growth potential and reduce water available for discharge to streams -Best controlled with conservation tillage & mulch VEG MULCH: mulch used to cover soil surface to control evaporation, erosion, temp, and weeds ex straw leaves crop residues ---Great but expensive and labor intensive, so best used for small areas & high value crops ex gardens, landscaping beds, cut flowers, berries, fruit trees ---Easier to produce a mulch "in place" by growing a cover crop --Reduce soil borne disease by splashing water, provide clean path for foot traffic, reduce weed growth, moderate soil temp (esp good for overheating in summer), increase water infiltration, provide organic matter, encourage earthworm populations, reduce soil erosion PLASTIC MULCH: control evaporative water loss, and warm soil. opaque plastic can help control weeds --expensive, so used only with highest value crops --covered with tree bark mulch for longevity and aesthetic --irrigation tubes often installed under plastic CONS: no o matter/reduction of soil erosion, hard to remove, not pretty, not sustainable --->biodegradable mulch! CROP RESIDUE/CONSERVATION TILLAGE: conservation tillage leave high % of residues from previous crop on or near the surface --Stubble Mulch tillage: dry regions, wheat stubble/cornstalks are uniformly spread on soil surface, field is lightly tilled --No till
Water loss by Percolation and leaching
-Percolation losses are influenced by 1) amount of rainfall and distribution 2) runoff from the soil 3) evaporation 4) character of the soil 5) nature of vegetation HUMID REGION: water percolates in late winter and early spring, and there is a depletion of soil water in the summer. Plants can only grow bc of soil water stored from previous winter/spring SEMIARID REGION: water is stored in the soil during the winter months and is used to meet moisture deficit in the summer, but because of low rainfall, little runoff and essentially no percolation out of profile occurs ARID (irrigated) REGION: irrigation & a little precip allows for percolation which is essential to remove excess soluble salts. During summer, fall, and winter, stored water is depleted bc amount added is less than that removed by very high ET
Reasons to Enhance Soil Drainage
-Water saturated, poorly aerated soils are essential to wetlands, but for most land uses, this is a bad condition PERCHED WATER TABLE: water accumulating above an impermeable layer in soil profile 1)Engineering Problems: mud & low bearing strength make it difficult to operate machinery, recreation soils can withstand trampling better, poorly drained soils make uneven settling and wet basements in houses, high water table under roads lower soil strength and lead to damage from frost heaving 2)Plant Production: water-saturation is bad for most upland crops and forest species. bad for farm equipment, bad for aeration & resulting restricted root system, prevent waterlogging and excessive salt in irrigated arid regions PROS: 1) Increased strength and soil workability 2) Less frost heaving of foundations & pavements & plants 3) Enhanced rooting depth, growth, and productivity due to oxygen supply 4) Reduced levels of fungal disease 5) More rapid soil warming 6) Less production of methane and nitrogen gas 7) Removal of excess salts from irrigated soils and prevention of salt accumulation by capillary rise in areas of salty groundwater CONS: 1) Loss of wildlife habitat 2) Reduction in wetland benefits 3) ^^ Leaching of contaminants 4) ^^ loss of soil matter, leading to subsidence 5) Greater cost of damages when flooding occurs
Alternatives to Drain Fields
1) Mound Drain Field System constructs drain field above ground, relies on pumps to pump wastewater up to perforated pipes in the mound --First sand then covered with sandy soil then covered with loamy soil --Vegetation established to use some of water for evapotranspiration 2) Artificial wetlands 3) Self-contained composting toilet
Soil Properties Influencing Suitability for a Septic Drain Field
1) Saturated hydraulic conductivity that will allow wastewater to enter and pass through the soil profile rapidly enough to avoid backups but slowly enough to allow soil to purify effluent 2) well aerated to encourage microbial breakdown and destroy pathogens Disqualifying features: fragipans/clay pans, gleying in the upper horizons, too steep a slope, excessivley drained sand/gravel SUITABILITY RATING: Depends on soil properties that affect water movement and the ease of installation -->15% slope allows lateral movement of percolating water so that wastewater can seep to surface --low water table so you have plenty of aerated soil to purify wastewater PERC TEST: determines percolation rate expressed in mm of water entering soil per hour --Low percolation can be compensated by increasing length of drain field pipes --Too low or too high is bad
Global Stocks of Water
97% of water is found in ocean where water is salty & has an average residence time of several thousand years and only near-surface ocean layers take part in annual water cycling 1.7% of the water is in glaciers and ice caps of mountains with similarly long residence time (10k years) 1.7% is found in groundwater, most of which is more than 750 m underground and about half of which is saline and also has a long average residence time SHORT RESIDENCE TIME: surface layer of ocean, shallow groundwater, lakes/rivers, atmosphere (10 days), soil water (1 month)
Water Use Efficiency
Application Efficiency: amount of water allocated to irrigate a field to amount of water being used in transpiration --most systems are 10-30% efficient --Water loss to evaporation (covered pipes-effective but expensive) and leakage in reservoirs/canals/ditches into the soil (line with concrete) Field Water Efficiency: Water transpired by the crop/ water applied to the field x 100% --usually less than 50%, semiarid regions as low as 20-25%
Hydrologic Cycle
Driven by solar energy stimulating evaporation, water vapor forms clouds that can move around, after about 10 days, pressure and temp differences in the atmosphere cause water vapor to condense into liquid droplets or solid particles which return to the earth as precipitation -65% of water falling on land is bound as soil moisture, most of which is transpired by plants or drained into groundwater -Surface runoff and groundwater seepage enter streams and rivers that flow into oceans
Sprinkler Systems
Ex: Center Pivot, Movable Pipe, Solid Set, moderate price, 60-70% field efficiency, level to moderately sloping, not too clayer soils -Water is sprayed through the air, which 1) causes water loss and 2) cools off & aerates water Con: wet leaves may increase incidence of fungal diseases in grapes, fruit trees, roses WATER CONTROL: water delivered at a rate less than infiltration capacity of the soil, if infiltration capacity is slow, runoff and erosion can be problems, better control over application rates = higher efficiency esp. on coarse soils, can be computer-controlled and callibrated & can deliver pesticides or fertilizers -Requires large pressure pumps, specialized pipes & nozzles
Surface Irrifation
Ex: basin, flood, furrow. cheap, 20-50% Field efficiency, good for level land, not too sandy or rocky -Water brought to fields in supply ditches or gated pipes -Sandy soils: leaching loss of water and chemicals is a problem at upper end of the field -Clayer soils: erosion, runoff, and waterlogging are problems at the lower end LEVEL BASIN: used for paddy rice, not suitable with coarse soils, build terraces, inexpensive, hard to control leaching and runoff, much water loss
Microirrigation
Ex: drip, porous pipe, spitter, bubbler, 80-90% field efficiency, low labor, most expensive, can be used on any soil -Pipes deliver small amounts of water directly to root zone at a low rate optimizing soil water availability in root zone while leaving most of soil volume dry -Bubblers (small vertical standpipes) and spitters (microsprayers) require a small level basin be formed in the soil under each tree -Can largely automated -Produces healthier plants and high crop yields because plant is never stressed by low water or low aeration -High risk, bc if system stops working, everything goes to shit -Best used in scare water supply areas, and where high-valued plants such as fruit trees or veg are being grown
Irrigation Importance
FOOD PRODUCTION: irrigation increases productivity of land by 100-400%, helps keep global food supplies keep up with population growth, ag. irrigation is largest consumer of water --30-35% of water withdrawals for irrigation are returned to water sources (consumptive use) as opposed to 90-95% of industry water being returned to the source (non consumptive) LANDSCAPING: golf courses, flower beds, municipal parks, home lawns to maintain desired year-round lushness (not practical) FUTURE PROSPECTS: water supplies are slowly dwindling bc 1) increased competition for water from growing urbam water users 2) overpumping of aquifers = falling water tables 3) reduction of storage capacity of reservoirs by siltation 4) increased recognition of need to allow portion of river flows to go unused by irrigation
Water Balance
P = ET+SS+D P=precipitation ET=evapotranspiration SS=soil storage D=discharge
Movement of Chemicals in Drainage Water
Percolation dissolves and leaches inorganic/organic chemicals found in soil or on land surface --NITROGEN: leaching of nitrogen 1) depletes nitrogen for plant usage 2) eutrophication 3) contaminates groundwater with nitrates --More concern: human pathogens, various toxic substances, pesticides, hormones, drugs, leached chemicals from disposal sites --Fine, uniform soils allow chemicals to move slowly and allow them to be removed or destroyed while well-drained, coarse textured soils have contaminants move more quickly
Subsurface Drainage Systems
Purpose is to remove groundwater from within the soil and lower water table. Some places like florida and netherlands must pump to remove drainage water DEEP OPEN-DITCH DRAINAGE: ditch excavated to a depth below the water table --saturated water moves under positive pressure to ditch --present barriers to equipment --Only practical for sandy sols that ensure that water table will be lowered for a considerable distance from each ditch and ditches can be placed far apart BURIED PERFORATED PIPES (Drain Tiles): network of plastic pipes with perforations side down so soil wont fall into pipe and clog it, protected outlet so rodents wont enter --control structure installed at outlet of pipe to allow manager to control the flow of water out of the system, controlling water table, drainage, flow of chemicals, etc. BUILDING FOUNDATION DRAINS: burying perforated pipes alongside and slightly below foundation, slopwed to allow water to move rapidly to an outlet ditch or sewer MOLE DRAINAGE: mole drain system can be created by pulling a pointed shank followed by an attached bullet-shaped steel plug about 7-10 cm in diameter through the soil at the desired depth. The compressed wall channel thus formed provides a pathway for the removal of excess water, similar to a buried pipe. --inexpensive, but efficient only in fine-textured soils
Effects on Evapotranspiration
SOIL MOISTURE: evap. from soil surface is determined largely by soil surface wetness. mostly upper 15-25 cm of soil provides water for surface evaporation, and upward capillary movement will soon dry out surface soil reducing further evaporation loss --> Plant roots that penetrate deep into soil will allow water loss by evapotranspiration to occur from subsoil layers. -----> esp important in regions with wet/dry seasons PLANT WATER STRESS: for dense veg. growing in soil well supplied with water, ET=PET. When soiil water content is less than optimal, plant will not be able to withdraw water from the soil fast enough to satisfy PET, causing plants to lose turgor and wilt ---Under these dry conditions, ET<PET and plant experiences water stress where they close their stomata stopping plant growth and reducing evaporative cooling --------> reduction of evaporative cooling allows from infrared detection of water stress ---PET-ET = water deficit PLANT CHARACTERISTICS: leaf area per unit land area (leaf area indiex LAI) ^^,absorbed radiation by foliage ^^^, less water will reach soil to promote evaporation --LAI will fluctuate with crop cycles (0 at planting, 3-5 at flowering, 0 at harvest) --Perennial vegetation & leaf litter block sunlight from soil, and block evaporation throughout the year --Other plant characteristics: rooting depth, length of life cycle, leaf morphology --Ex: mature trees compete with ground level vegetation for water supply WATER USE EFFICIENCY: expressed in plant dry matter produced/water transpired (T efficiency) OR per unit of water lost by evapotranspiration (ET efficiency) -Humid regions with more water need less water to produce each kg of grain because evaporative demand is much lower when compared to arid regions ET EFFICIENCY: is highest where plant density and other growth factors minimize the proportio of ET --Higher yielding plants send their roots deeper and produce a denser canopy (higher LAI) allowing less solar radiation and less evaporation from soil surface --In general, closer plant spacing & fertilization & selecting more vigorous plants increases water use efficiency --------> NOTE: if irrigation water is not available and rainfall period is short, one should apply this principle with caution bc ^^ of ET by more vigorously growing plants may deplete stored soil water => water stress or plant death HYDRAULIC REDISTRIBUTION: Plant roots can release water back into the soil. During the night when atmospheric humidity is high and leaf stomata are closed, the lack of pull from transpiration ^ water potential in roots, and if it gets higher than that of surrounding soil, the water will begin to move from the root to the soil --Hydraulic redistribution or hydraulic lift is allowing plant roots to move water form moist soil to dry soil EX: trees take water moisture from subsoil and transport it to surface soil --Can create "self-irrigating" agroecosystems where rows of deeprooted "water donor" plants are alternated with water-receiving areas planted with shallow rooted but high value vegetable crops.
Soil-Plant-Atmosphere Continuum
SPAC: major component of overall hydrologic cycle ties together interception, surface runoff, percolation, drainage, evaporation, plant water uptake, ascent of water to plant leaves, and transpiration of water from leaves back into the atmosphere -Water potentials: same basic principles that govern retention and water flow control the movement of water whether in soil,plants, or atmosphere. ex: If a plant wants to absorb water from the soil, the water potential must be lower (greater negative value) in the plant root than the soil ex: water movement up root and stem into leaf cells is in response to differences in water potential, as is the movement from leaf surfaces to the atmosphere. ex: soil water is -50 kPa, root is -70 kPa, stem is -75 kPa, upper stem is -85 kPa, leaf surface is -500 kPa, and atmosphere is -20000 kPa -Water encounters major resistance as it crosses cell membranes at root-soil interface and again at leaf-atmosphere interface, so there are 2 primary factors determine whether plants are well supplied with water: 1. rate at which water is supplied by the soil to the absorbing roots 2. rate at which water is transpired from the plant leaves
Effect of Vegetation and Soils on Infiltration
Type of Veg: grasslands and dense forests protect soil structure from beating action of raindrops & encourage infiltration and discourage runoff, even diff. between species on grasslands makes a difference Stem Flow: water trickling down leaves, twigs, brances, tree trunks; can funnel water into crop row. must be considered in studying hydrology and nutrient cycling in many plant ecosystems Soil Management: encouraging infiltration rather than runoff is a major objective of soil & water management --make small furrows in bare soil to allow more time for the water to infiltrate --plant cover crops to open root channels encourage earthworms and protect soil surface structure ------->but cover crops will depend on soil storage water --minimize soil structure by minimizing compaction Urban Watersheds: ^^ compaction ^^runoff ^^burden on ecosystems. also ^^ impervious surfaces ^^runoff^^burden, also storm sewers and street gutters rush excess water off the land --Use low-impact urban design w permeable pavers which allow grass to grow and water to infiltrate and w rain gardens Soil Properties: loose and open soils (sands, well-granulated) will encourage infiltration while heavy clay soils with unstable structures resist infiltration and encourage runoff, soil animals enhance infiltration and reduce runoff losses
Control of Transpiration
UNWANTED VEG: weeds use soil water that should be used for establishment & growth of desired veg. Historically controlled by light soil tillage, but this can damage roots of desired veg, and expose bare soils, which increases E and incourages runoff & erosion HERBICIDES: require less labor & energy, allows soil to be undisturbed, and allows plant residues to cover the soil surface --Cons: high material costs, weed resistance, damage to desired plants, toxicity ALTERNATIVE WEED CONTROLS: biological controls like targeting bugs/bacteria, controlled fires, mowing/grazing, cover crops FALLOW IN DRYLAND CROPPING: alternating bare fallow (unvegetated period) can help conserve soil moisture in some low-rainfall environmnets, and can increase yields post-fallow --Cons: negative soil o matter balance, wind erosion BEST PRACTICES: cropping systems with conservation tillage, keeping soil vegetated with a variety of crops
Surface Drainage Systems
Used where landscape is nearly level and soils are fine-textured with slow percolation SURFACE DRAINAGE DITCHES: construction of shallow ditches or swales with gentle side slopes that are sloped to permit interception of water as it runsoff LAND SMOOTHING: high spots are cut down and depressions are filled in allowing excess water to move over soil surface at controlled rate to an outlet ditch and natural drainage channel
Soil Water Vocab
WATER TABLE: upper surface of zone of saturation of water - 1-10 m below surface in humid regions, 100-1000's of m below surface in arid regions, essentially at land surface in swamps GROUNDWATER: water within saturated zone VADOSE ZONE: unsaturated zone above the water table AQUIFERS: porous geological materials that groundwater seeps through until it is discharged into springs and streams UNCONFINED AQUIFER: shallow water-bearing layer that is not separated from the soil surface by any overlying impermeable layer -->replenished annually -->commonly provide water for farm and rural dwellings
Septic Tank Drain Fields
When developing, the land must be approved by a soil scientist as having acceptable drainage for a septic tank drain field Improperly sited septic tank drain fields contributes greatly to pollution of groundwater and streams Can contribute to groundwater! Septic tank: waste water flows to large underground concrete box, solid waste is decomposed by microbial action and volume is reduced, so many years may pass before septic tank becomes too full and accumulated sludge/septage has to be pumped out --depends on active microorganisms, so should avoid introducing harmful chemicals or non-biodegradable materials Drain Field: waste water with organic particles & dissolved chemicals & microogs is directed to one or more buried pipes blanketed in gravel and buried in trenches, perforated on the bottom ---soil properties must 1)keep effluent out of sight and contact with people 2) purify the effluent and 3) conduct the purified effluent to the groundwater ***********Releases nitrates to groundwater!!
Watershed
an area of land drained by a single system of streams and bounded by ridges that separate it from adjacent watersheds --Precip falling on a watershed is stored in the soil, returned to the atmosphere, or discharged as surface or subsurface runoff
