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| WATER |
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| What is the role of water in plants? |
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| Water availability to turfgrasses from rainfall and soil storage is rarely synchronized with plant needs in either time or quantity. |
| Water problems very from absolute desiccation to submersion and correction of these problems must be addressed by judiciously applied irrigation or drainage. |
| Water requirement (WR) - the amount of water from rainfall and irrigation necessary to meet specific production or performance needs. |
| Water use the efficiency (WUE.) - quantity of water per unit production; generally expressed as grams of H2O to produce 1 g dry matter. |
| Water use rates are expressed in terms of inches or millimeters used per day. This is often referred to as evapotranspiration (ET). |
| Water requirement and water use efficiency by turfgrass are determined in relation to performance and quality standards, rather than yield standards. |
| With this in mind, it is implied that it is a minimum value that varies with the type of turf or intended use. |
| The water use rate may be influenced by soil moisture availability, the degree of water demand, by the atmosphere, and by cultural management practices. |
| The minimum values will very with the type of turf used or required. |
| The turf water use rate may be influenced by soil moisture availability, the degree of water demand, by the atmosphere, and bicultural practices. |
| Drought terminology |
| Drought - a condition caused by a period of dry weather, sufficiently prolonged to caused plant damage and water supply shortages, usually defined as a cumulative precipitation deferring from normal, accompanied by above normal atmospheric demands. |
| Drought has various meanings among scientists, turf managers, and lay personnel. |
| Drought resistance is a general term encompassing mechanisms in which plants withstand periods of dry weather. These mechanisms are comprised of drought avoidance and drought tolerance |
| Drought avoidance is the ability of plant to avoid tissue damaging water deficits even while growing in the drought environment. |
| A positive tissue water balance is thus maintained and excludes the stress. |
| Drought avoidance may be a function of increased root depth and root water uptake properties, and reduce ET |
Drought tolerance is the ability of the turf to endure low (more negative) water potentials caused by drought. |
| Drought-tolerance mechanisms include osmoregulation and desiccation tolerance achieve via protoplasm resistance. |
| Considerations of turfgrass water requirement should be "what are the minimum required by turfgrass for the performance level desired?" |
| You can also ask "how much performance can be obtained from a given amount of water?" |
| Evaporative demand in water use |
| The driving force for evaporative demand is solar radiation. Amounts and effectiveness of solar radiation are functions of climate, season, altitude, and latitude. |
| The first consideration in determining water requirements is to establish basic patterns of evaporative demand and determine how that demand is translated into ET losses from turf whose water use is not restricted in any other way. |
| An obvious measurement of evaporative demand is to expose a free water surface and measure water loss. |
| Most data are available from the US Weather Service Class A pan. |
| This is a 122 cm in diameter and 25 cm deep search your container supported 15 cm. |
| If conditions for the pan site are closely match to those for the crop, the correlation between pan loss and consumptive water use will be similar. |
| Consumptive use, however is usually less than the average loss for Class A pan, varying from 50 to 90% with stage of growth, crop, and other modifying factors. |
| It is important to know if pan evaporimeters use the Class A standards. If not then pan reference values may be seriously biased, usually are upwards. |
| What is water potential? |
| Water potential is a tendency or pressure of water to move across a barrier. |
| Semipermeable membrane |
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Pure water at atmospheric pressure is zero.  = 0 |
| Increasing pressure will raise the water potential. |
| Water potential of a solution is generally negative. |
| Water will diffuse from high water potential to low. |
Water potential is made up of two or more components, pressure potential + osmotic potential.
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| g or the gravitational potential of water is not considered for turf. |
Pressure potential ( ) is due to the addition of pressure. |
| Tension (is the opposite of pressure) results in a negative pressure potential. |
Osmotic potential ( ) is due to the presence of a solute. |
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| What are some typical for plants? for leaves
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| Units - 1 MPa = 10 bars:1 MPa = 9.90 atmospheres |
in soil: = + +  Where
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| PLANT-ATMOSPHERIC CONTINUUM (SPAC)
Plants can be envisioned to be a straw connecting the atmosphere to the soil. The pull of the atmosphere (along with evaporative cooling) is what draws water out of the plants. Relative Humidity Air What is the driving force for transpiration? The water potential gradient between the plant and the atmosphere. How can we determine ET? measurements in the field are difficult to do and may be site specific. Prediction from weather data 1. aerodynamic-profile method 1. aerodynamic works by measuring vapor pressures and other weather data at two different heights and using this information to predict ET 2. energy budget method H = energy going into heating the air Problems with using this method 3. Penman method is a combination of 1 and 2. Penman equation uses method #1 to estimate H. Then uses the energy budget and assumes G=0 which does introduce large errors. value of Penman equation: 1. It estimates potential evaporation from a free water surface. 4. Empirical method - works well on a season basis. Kc = Crop coefficients are determined by measuring in the field. |
| Crop Coefficients - what are they?
Crop Coefficient - a fudge factor to relate potential ETo to actual ET. Crop coefficients can also be used to relate measured ET to expected ET. Points to make: 2) Considerable variation between cultivars of the same species. Factors that affect ET rates of turfgrasses 1) Mowing height or leaf area. ET for turf mowed at 3/4” = 4.75 mm Writing exercise: Explain the above phenomenon? If Rn is equal what must be happening? LEt is larger for 2” HOC so H or G must smaller. 2) High canopy resistance reduces ET 3) Low leaf area Use of ET data How is ET data used? Provides no information on the water status of the soil. What is importance of knowing water status of the soil? Soil is made up ideally of 50% solids, 25% air, 25% water. When the water starts replacing the air for too long a time, that creates poor growing conditions, root death, anaerobic conditions, etc. How can you determine the water content of the soil? TDR is a new technique that measures volumetric water content. I predict that these will become widely used by superintendents in 3-5 years. Soil Moisture status provides good reference on water used by turf during the day. Can be part of an overall water management strategy. Question: How do you as manager decide how much water to apply? Use of ET in deficit irrigation Summarize value of deficit irrigation strategy 1) Water conserving strategy |
| Water use models |
| Penman estimates |
| Penman estimates for water use were derived as a means of calculating loss from a free water surface (Penman, 1948). |
| Water loss is estimated by an equation combining energy balance and mass transfer principles. |
| Data required for the Penman model include wind speed, net radiation, temperature and vapor pressure deficits (VPD). |
| Local calibration of the Penman model is often necessary because of daily patterns of wind, temperature, VPD and net radiation (Rn) are represented as a single numerical ET value. |
| The Penman model is applicable to humid and dry climates alike for estimating a potential (reference) ET. |
| Penman values are adjusted by a crop coefficient (KC) to estimate actual potential ET losses from a particular crop involved. |
| It is important to realize that a KC value is a fractional percentage of a basis (reference) value. |
| Blaney and Criddle model |
| Once widely used to estimate evaporative demand in the Western USA. |
| Evaporative demand is determine using monthly average temperatures and the percentage of annual daylight hours occurring in that month. |
| It derives a generalized estimate of water use which is adjusted by a crop coefficients. |
| The TORO Company used the Blaney-Criddle formula in 1966 to estimate potential irrigation needs across the entire country. |
| Jensen-Haise equation |
| More precise then the Blaney-Criddle procedure and much simpler than the Penman method yet derides values very close to Penman over time. |
| The model relates reference ET to solar radiation and temperature using alfalfa as a reference crop. |
| This model is best used for periods that span a minimum of five days or so. |
| Jensen-Haise estimates are presented in daily public radio announcements are available from a "Hotline" number in Denver, Colorado. |
| This is made available to the public as an estimate to base watering programs as part of a community water conservation effort. |
