EVAPOTRANSPIRATION OF PARTIALLY VEGETATED SURFACES

Abstract

Latent heat flux equivalent to Evapotranspiration (ET) is the total amount of water lost via transpiration and evaporation from plant surfaces and the soil in an area where a crop is growing. Since 80-90% of precipitation received in semiarid and subhumid climates is commonly used in evapotranspiration, accurate estimations of ET are very important for hydrologic studies and crop water requirements. ET determination and modelling is not straightforward due to the natural heterogeneity and complexity of agricultural and natural land surfaces. In evapotranspiration modelling it is very common to represent vegetation assuming a single source of energy flux at an effective height within the canopy. However, when crops are sparse, the single source/sink of energy assumption in such models is not entirely satisfied. Improvements using multiple source models have been developed to estimate ET from crop transpiration and soil evaporation. Soil evaporation on partially vegetated surfaces over natural vegetation and orchards includes not only the soil under the canopy but also areas of bare soil between vegetation that contribute to ET. Soil evaporation can account for 25-45% of annual ET in agricultural systems. In irrigated agriculture, partially vegetated surfaces include fruit orchards (i.e. apples, oranges, vineyards, avocados, blueberries, and lemons among others), which cover a significant portion of the total area under irrigation. In semiarid regions, direct soil evaporation from sparse barley or millet crops can account for 30% to 60% of rainfall (Wallace et al., 1999). On a seasonal basis, sparse canopy soil evaporation can account for half of total rainfall (Lund & Soegaard, 2003). Allen (1990) estimated the soil evaporation under a sparse barley crop in northern Syria and found that about 70% of the total evaporation originated from the soil. Lagos (2008) estimated that under irrigated maize conditions soil evaporation accounted for around 26-36% of annual evapotranspiration. Under rain-fed maize conditions annual evaporation accounted for 36-39% of total ET. Under irrigated soybean the percentage was 41%, and under rainfed soybean conditions annual evaporation accounted for 45-47% of annual ET. Massman (1992) estimated that the soil contribution to total ET was about 30% for a short grass steppe measurement site in northeast Colorado. In a sparse canopy at the middle of the growing season, and after a rain event, more than 50% of the daily ET corresponds to directly soil evaporation (Lund & Soegaard, 2003). Soil evaporation can be maximized under frequent rainfall or irrigation events, common conditions in agricultural systems for orchard with drip or micro sprinklers systems. If some of this unproductive loss of water could be retained in the soil and used as transpiration, yields could be increased without increased rainfall or the use of supplemental irrigation (Wallace et al., 1999). The measurement and modelling of soil evaporation on partially vegetated surfaces is crucial to estimate how much water is lost to the atmosphere via soil evaporation. Consequently, better water management can be proposed for water savings. Partially vegetated surface accounts for a significant portion of land surface. It occurs seasonally in all agricultural areas and throughout the year in or chard and natural land covers. Predictions of ET for these conditions have not been thoroughly researched. In Chile, agricultural orchards with partially vegetated surfaces include apples, oranges, avocados, cherries, vineyards, blueberries, and berries, among others. According to the agricultural census (INE, 2007) the national orchard surface covers more than 324,000 ha, representing 30% of the total surface under irrigation. Similar to the Shuttleworth and Wallace (1985), Choudhury and Monteith (1988) and Lagos (2008) models, the modelling of evapotranspiration for partially vegetated surfaces can be accomplished using explicit solutions of the equations that define the conservation of heat and water vapor fluxes for partially vegetated surfaces and soil. Multiple-layer models offer the possibility to represent these conditions to solve the surface energy balance and consequently, estimate evapotranspiration. Modelling is essential to predict long-term trends and to quantify expected outcomes. Since ET is such a large component of the hydrologic cycle in areas with partially vegetated surfaces, small changes in the calculation of ET can result in significant changes in simulated water budgets. Thus, good data and accurate modelling of ET is essential for predicting not only water requirements for agricultural crops but also to predict the significance of irrigation management decisions and land use changes to the entire hydrologic cycle. Currently, several methods and models exist to predict natural environments under different conditions. More complex models have been developed to account for more variables affecting model performance. However, the applicability of these models has been limited by the difficulties and tedious algorithms needed to complete estimations. Mathematical algorithms used by multiple-layer models can be programmed in a software package to facilitate and optimize ET estimation by any user. User-friendly software facilitates the use of these improved methods; users (i.e. students) can use the computer model to study the behaviour of the system from a set of parameters and initial conditions. Accordingly, in this chapter, a review of models that estimate ET for partially covered surfaces that occur normally in agricultural systems (i.e. orchards or vineyards) is presented, and the needs for further research are assessed.Latent heat flux equivalent to Evapotranspiration (ET) is the total amount of water lost via transpiration and evaporation from plant surfaces and the soil in an area where a crop is growing. Since 80-90% of precipitation received in semiarid and subhumid climates is commonly used in evapotranspiration, accurate estimations of ET are very important for hydrologic studies and crop water requirements. ET determination and modelling is not straightforward due to the natural heterogeneity and complexity of agricultural and natural land surfaces. In evapotranspiration modelling it is very common to represent vegetation assuming a single source of energy flux at an effective height within the canopy. However, when crops are sparse, the single source/sink of energy assumption in such models is not entirely satisfied. Improvements using multiple source models have been developed to estimate ET from crop transpiration and soil evaporation. Soil evaporation on partially vegetated surfaces over natural vegetation and orchards includes not only the soil under the canopy but also areas of bare soil between vegetation that contribute to ET. Soil evaporation can account for 25-45% of annual ET in agricultural systems. In irrigated agriculture, partially vegetated surfaces include fruit orchards (i.e. apples, oranges, vineyards, avocados, blueberries, and lemons among others), which cover a significant portion of the total area under irrigation. In semiarid regions, direct soil evaporation from sparse barley or millet crops can account for 30% to 60% of rainfall (Wallace et al., 1999). On a seasonal basis, sparse canopy soil evaporation can account for half of total rainfall (Lund & Soegaard, 2003). Allen (1990) estimated the soil evaporation under a sparse barley crop in northern Syria and found that about 70% of the total evaporation originated from the soil. Lagos (2008) estimated that under irrigated maize conditions soil evaporation accounted for around 26-36% of annual evapotranspiration. Under rain-fed maize conditions annual evaporation accounted for 36-39% of total ET. Under irrigated soybean the percentage was 41%, and under rainfed soybean conditions annual evaporation accounted for 45-47% of annual ET. Massman (1992) estimated that the soil contribution to total ET was about 30% for a short grass steppe measurement site in northeast Colorado. In a sparse canopy at the middle of the growing season, and after a rain event, more than 50% of the daily ET corresponds to directly soil evaporation (Lund & Soegaard, 2003). Soil evaporation can be maximized under frequent rainfall or irrigation events, common conditions in agricultural systems for orchard with drip or micro sprinklers systems. If some of this unproductive loss of water could be retained in the soil and used as transpiration, yields could be increased without increased rainfall or the use of supplemental irrigation (Wallace et al., 1999). The measurement and modelling of soil evaporation on partially vegetated surfaces is crucial to estimate how much water is lost to the atmosphere via soil evaporation. Consequently, better water management can be proposed for water savings. Partially vegetated surface accounts for a significant portion of land surface. It occurs seasonally in all agricultural areas and throughout the year in or chard and natural land covers. Predictions of ET for these conditions have not been thoroughly researched. In Chile, agricultural orchards with partially vegetated surfaces include apples, oranges, avocados, cherries, vineyards, blueberries, and berries, among others. According to the agricultural census (INE, 2007) the national orchard surface covers more than 324,000 ha, representing 30% of the total surface under irrigation. Similar to the Shuttleworth and Wallace (1985), Choudhury and Monteith (1988) and Lagos (2008) models, the modelling of evapotranspiration for partially vegetated surfaces can be accomplished using explicit solutions of the equations that define the conservation of heat and water vapor fluxes for partially vegetated surfaces and soil. Multiple-layer models offer the possibility to represent these conditions to solve the surface energy balance and consequently, estimate evapotranspiration. Modelling is essential to predict long-term trends and to quantify expected outcomes. Since ET is such a large component of the hydrologic cycle in areas with partially vegetated surfaces, small changes in the calculation of ET can result in significant changes in simulated water budgets. Thus, good data and accurate modelling of ET is essential for predicting not only water requirements for agricultural crops but also to predict the significance of irrigation management decisions and land use changes to the entire hydrologic cycle. Currently, several methods and models exist to predict natural environments under different conditions. More complex models have been developed to account for more variables affecting model performance. However, the applicability of these models has been limited by the difficulties and tedious algorithms needed to complete estimations. Mathematical algorithms used by multiple-layer models can be programmed in a software package to facilitate and optimize ET estimation by any user. User-friendly software facilitates the use of these improved methods; users (i.e. students) can use the computer model to study the behaviour of the system from a set of parameters and initial conditions. Accordingly, in this chapter, a review of models that estimate ET for partially covered surfaces that occur normally in agricultural systems (i.e. orchards or vineyards) is presented, and the needs for further research are assessed.Latent heat flux equivalent to Evapotranspiration (ET) is the total amount of water lost via transpiration and evaporation from plant surfaces and the soil in an area where a crop is growing. Since 80-90% of precipitation received in semiarid and subhumid climates is commonly used in evapotranspiration, accurate estimations of ET are very important for hydrologic studies and crop water requirements. ET determination and modelling is not straightforward due to the natural heterogeneity and complexity of agricultural and natural land surfaces. In evapotranspiration modelling it is very common to represent vegetation assuming a single source of energy flux at an effective height within the canopy. However, when crops are sparse, the single source/sink of energy assumption in such models is not entirely satisfied. Improvements using multiple source models have been developed to estimate ET from crop transpiration and soil evaporation. Soil evaporation on partially vegetated surfaces over natural vegetation and orchards includes not only the soil under the canopy but also areas of bare soil between vegetation that contribute to ET. Soil evaporation can account for 25-45% of annual ET in agricultural systems. In irrigated agriculture, partially vegetated surfaces include fruit orchards (i.e. apples, oranges, vineyards, avocados, blueberries, and lemons among others), which cover a significant portion of the total area under irrigation. In semiarid regions, direct soil evaporation from sparse barley or millet crops can account for 30% to 60% of rainfall (Wallace et al., 1999). On a seasonal basis, sparse canopy soil evaporation can account for half of total rainfall (Lund & Soegaard, 2003). Allen (1990) estimated the soil evaporation under a sparse barley crop in northern Syria and found that about 70% of the total evaporation originated from the soil. Lagos (2008) estimated that under irrigated maize conditions soil evaporation accounted for around 26-36% of annual evapotranspiration. Under rain-fed maize conditions annual evaporation accounted for 36-39% of total ET. Under irrigated soybean the percentage was 41%, and under rainfed soybean conditions annual evaporation accounted for 45-47% of annual ET. Massman (1992) estimated that the soil contribution to total ET was about 30% for a short grass steppe measurement site in northeast Colorado. In a sparse canopy at the middle of the growing season, and after a rain event, more than 50% of the daily ET corresponds to directly soil evaporation (Lund & Soegaard, 2003). Soil evaporation can be maximized under frequent rainfall or irrigation events, common conditions in agricultural systems for orchard with drip or micro sprinklers systems. If some of this unproductive loss of water could be retained in the soil and used as transpiration, yields could be increased without increased rainfall or the use of supplemental irrigation (Wallace et al., 1999). The measurement and modelling of soil evaporation on partially vegetated surfaces is crucial to estimate how much water is lost to the atmosphere via soil evaporation. Consequently, better water management can be proposed for water savings. Partially vegetated surface accounts for a significant portion of land surface. It occurs seasonally in all agricultural areas and throughout the year in or chard and natural land covers. Predictions of ET for these conditions have not been thoroughly researched. In Chile, agricultural orchards with partially vegetated surfaces include apples, oranges, avocados, cherries, vineyards, blueberries, and berries, among others. According to the agricultural census (INE, 2007) the national orchard surface covers more than 324,000 ha, representing 30% of the total surface under irrigation. Similar to the Shuttleworth and Wallace (1985), Choudhury and Monteith (1988) and Lagos (2008) models, the modelling of evapotranspiration for partially vegetated surfaces can be accomplished using explicit solutions of the equations that define the conservation of heat and water vapor fluxes for partially vegetated surfaces and soil. Multiple-layer models offer the possibility to represent these conditions to solve the surface energy balance and consequently, estimate evapotranspiration. Modelling is essential to predict long-term trends and to quantify expected outcomes. Since ET is such a large component of the hydrologic cycle in areas with partially vegetated surfaces, small changes in the calculation of ET can result in significant changes in simulated water budgets. Thus, good data and accurate modelling of ET is essential for predicting not only water requirements for agricultural crops but also to predict the significance of irrigation management decisions and land use changes to the entire hydrologic cycle. Currently, several methods and models exist to predict natural environments under different conditions. More complex models have been developed to account for more variables affecting model performance. However, the applicability of these models has been limited by the difficulties and tedious algorithms needed to complete estimations. Mathematical algorithms used by multiple-layer models can be programmed in a software package to facilitate and optimize ET estimation by any user. User-friendly software facilitates the use of these improved methods; users (i.e. students) can use the computer model to study the behaviour of the system from a set of parameters and initial conditions. Accordingly, in this chapter, a review of models that estimate ET for partially covered surfaces that occur normally in agricultural systems (i.e. orchards or vineyards) is presented, and the needs for further research are assessed.