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Irrigation guidelines based on historical weather data in the Lower Rio Grande Valley of Texas
Agricultural Water Management 76 (2005) 1–7 www.elsevier.com/locate/agwat
Irrigation guidelines based on historical weather data in the Lower Rio Grande Valley of Texas Juan Enciso, Bob Wiedenfeld * Texas A&M University Research and Extension Center, 2415 E. Highway 83, Weslaco, TX 78596, USA Accepted 18 February 2005 Available online 14 March 2005
Abstract Irrigation guidelines based on historical data would be an important tool for planning irrigation, and may be considered as a management strategy depending on the weather variability. This paper analyzes variability in evapotranspiration (ET) patterns over a 9-year period between years and between different months. ET and rainfall data were collected with automatic weather stations and coefficients of variability were determined. The coefficient of variability for these data averaged less than 15% indicating the possibility of using these irrigation guidelines. The coefficient for the relationship between Pennman Monteith reference ET and class-A pan evaporation was found to be 0.7. Irrigation guidelines for sugarcane and citrus based on historical data were developed for irrigation management. # 2005 Elsevier B.V. All rights reserved. Keywords: Evapotranspiration; Pan evaporation
1. Introduction There are several networks providing weather information in Texas, including the National Weather Service, Mesonet, the Texas ET Network, and the Crop Weather Program for Southwest Texas. The National Weather Service and the Mesonet provide broadly dispersed regional weather data across the state while the Texas ET Network and the Crop Weather Program provide evapotranspiration (ET) data specific for local * Corresponding author. Tel.: +1 9569685585; fax: +1 9569695620. E-mail address: [email protected] (B. Wiedenfeld). 0378-3774/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2005.02.009
2
J. Enciso, B. Wiedenfeld / Agricultural Water Management 76 (2005) 1–7
conditions. The information provided by these networks is reported daily and can be accessed through the internet. Many years of data have been collected by these networks. Farmers are being trained in the use of ET for irrigation management, however, irrigation scheduling based on actual ET data has been adopted slowly by a limited number of growers. In Southwest Texas farmers have preferred the use of irrigation sensors such as capacitance probes and Watermark1 sensors to schedule irrigation. Generally, growers manage their irrigation by determining when to initiate water application as indicated by these sensors, and by making certain assumptions regarding soil water availability and depletion levels in the soil. Farmers give several reasons for why they do not use daily ET including: (1) ET does not take into account crop stress such as diseases; (2) they already know crop water requirements and these are constant year-to-year; (3) they want to irrigate based on direct observation of the condition of the plant; and (4) soil water balance is difficult for their particular soil and the internet is difficult to access. In some locations, general guidelines based on standard crop water requirements and number of irrigations depending on soil type have been adopted. These guidelines indicate the number of irrigations and amounts by crop, and were developed using average historical ET data. Examples include crops such as sugarcane that has recommendations provided in the South Texas Sugarcane Production Handbook (Rozeff, 1998), or citrus producers that have knowledge based on experience regarding seasonal irrigation requirements. The climate of the Lower Rio Grande Valley is identified as semiarid according to criteria established by USNESCO (1977) which identifies climatic zones according to an aridity index (precipitation divided by reference ET). The most variable weather parameter and the most unpredictable is rainfall. The coefficient of variability for rainfall in humid areas has been estimated to be 10 to 20%, whereas in semi-arid areas it may vary from 20 to 30% (Bhuiyan and Undan, 1990). Temperature variability declines as one moves from a temperate to a tropical climate. The Lower Rio Grande Valley is located at 308 N latitude just north of the tropic of cancer (23.58 N latitude), and extends from the coast to 150 km inland from the Gulf of Mexico. The objectives of this study were to evaluate the coefficient of variability in ET over a period of 9 years, to use average ET to develop irrigation guidelines for sugarcane and citrus, and to determine the relationship between ET and class-A pan evaporation in the subtropical, semiarid Lower Rio Grande Valley of Texas.
2. Materials and methods Nine years of weather data were collected from three weather stations in the Lower Rio Grande Valley of Texas. Stations were located on three farms of the Texas A&M Research and Extension Center at Weslaco between 2.5 and 5.3 km from each other. Weather stations were separated by distances comparable to situations where farmers use weather data from stations located in their proximity to estimate ET. Weather stations were Campbell Scientific model Met Data 1 or ET106 containing CR10X data loggers with the following sensors: Vaisala CS500 temperature and relative humidity probe, Met One 034A windset, Li-Cor LI-200X pyranometer, and Texas Instruments TE525 tipping bucket rain gage. Data collected were hourly average and daily maximum and minimum temperature and relative
J. Enciso, B. Wiedenfeld / Agricultural Water Management 76 (2005) 1–7
3
humidity, daily average wind speed, cumulative daily solar radiation and rainfall. ET was calculated daily using the Penman-Monteith ASCE standardized reference equation (Walter et al., 2000). Evaporation from a U.S. Weather Bureau evaporation pan (class-A pan, American Meteorological Society, 2000) was manually recorded daily at one location. Mean and standard deviation were calculated for data totaled by month for each station. When any data was missing, that month was not included in the calculations. Monthly sugarcane and citrus ET was calculated by using crop coefficients recommended by Allen et al. (1998). The sugarcane crop coefficient curve was adjusted to local growing conditions based on sugarcane water use determined by Salinas and Namken (1977). Irrigation guidelines were developed using the Harlingen silty fine clay, Raymondville clay loam and Rio Grand silt loam soils with water holding capacities of 91, 122, and 152 mm per 60 cm of soil depth, respectively, that represent a wide range of irrigation depths.
3. Results Average ET, pan evaporation and rainfall data is presented in Fig. 1. Pan evaporation was always higher than reference ET, and ET higher than rainfall. Highest average monthly rainfall received was during September with 127 mm followed by October with 86 mm, and this is one of the reasons that little irrigation is required from September through December. The wettest part of the year is between August and October in this region. The development of season climate forecasting based on the El Nin˜ o Southern Oscillation (ENSO) phenomenon has introduced the possibility of crop management based on seasonal forecasts (Stone et al., 1996). Management could be adjusted according to probable weather trends such as rainfall. The coefficient of variability for rainfall in this study was highly variable. The average of the lowest coefficient of variability of the three weather stations was 63% for the month of May. Rainfall predictions are beyond the scope
Fig. 1. Average monthly class-A pan evaporation, Penman-Monteith reference ET and rainfall for 1995–2003 in the Lower Rio Grande Valley of Texas.
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Fig. 2. Coefficient of variability (CV) for monthly reference ET during a 9-year period at three locations and averaged across location in the Lower Rio Grande Valley of Texas.
of this study, but future research could associate seasonal rainfall with the Southern Oscillation Index (SOI) or Sea Surface Temperatures (SST). The coefficient of variability for reference ET varied between the three stations, and also over time at each station (Fig. 2). The Annex station showed the least variability, while higher coefficients of variability were observed at the Center and Hiler Stations. Over time the coefficient of variability averaged 14% from February to August then increased from September through January but always remained below 25% with the exception of December that was 38% (Fig. 2). This would indicate that irrigation guidelines for the first 8 months are more accurate but then become more variable, probably varying by as much as 30% after September. It is important to note that lower coefficients of variability were observed during the months when ET demand, and therefore, irrigation requirements would be highest. In September when variability starts to increase, ET begins to decline (Fig. 3). This also coincides with the time when farmers in the Lower Rio Grande Valley attempt to deplete soil moisture by limiting irrigation for sugarcane to promote sugar
Fig. 3. Percent of annual reference ET by month, and coefficient of variability (CV) for monthly reference ET for a 9-year period in the Lower Rio Grande Valley of Texas.
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5
Fig. 4. Ratio between reference ET and class-A pan evaporation for 1995-2003 at the Texas A&M University Research and Extension Center weather station, Weslaco, TX.
accumulation. The importance of having accurate data after September is reduced because besides declining ET demand, more rain is normally received during this period. Some farmers also like to base their predictions on pan evaporation. Pan evaporation was always higher than reference ET, with the ratio between reference ET and class-A pan ranging between 0.65 and 0.85 (Fig. 4). This indicates that a pan evaporation ratio of 0.7 may be used to estimate ET. Table 1 presents the number of irrigation required for sugarcane for the three types of soils assuming a management allowable depletion of 60% and using average ET and rainfall. Only one irrigation was required during November for sugarcane and it was for the soils with lower water holding capacity. The soil with holding capacity greater than 152 mm did not require irrigation for sugarcane after September. A similar trend was observed with citrus, which required one irrigation during November. However, the soil Table 1 Monthly average reference ET, rainfall, crop coefficients (Kc) and number of irrigations required for sugarcane for three soil types with different root depths in the Lower Rio Grande Valley of Texas Month
January February March April May June July August September October November December Total
ETo (mm)
Rain (mm)
Kc sugarcane
87 95 127 150 181 183 198 190 148 125 97 79
6 9 37 33 34 62 48 63 126 86 45 10
0.40 0.40 0.40 0.64 1.15 1.25 1.25 1.25 1.10 0.70 0.40 0.40
1660
560
Harlingen fine clay (91 mm)
Raymondville clay loam (122 mm)
Rio Grande Silt loam (152 mm)
1 1 1 1 2 2 2 1 0 0 1 0
0 1 1 1 2 1 1 1 0 0 1 0
0 1 1 1 1 1 1 1 0 0 0 0
12
9
7
6
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Table 2 Monthly crop coefficients (Kc) and number of irrigations required for mature citrus trees with 70% canopy cover with and without active ground cover or weeds Month
January February March April May June July August September October November December
Harlingen fine clay
Kc
Raymondville clay loam Rio Grande silt loam
With cover No cover With cover No cover With cover No cover
With cover No cover
0.75 0.75 0.75 0.75 0.70 0.70 0.70 0.70 0.75 0.75 0.75 0.75
Total
0.70 0.70 0.70 0.70 0.65 0.65 0.65 0.65 0.70 0.70 0.70 0.70
0 1 1 1 1 2 2 1 0 0 1 0
1 1 1 1 1 1 1 1 0 0 1 0
0 1 0 1 1 1 1 1 0 0 1 0
0 1 0 1 1 1 0 1 0 0 1 0
0 0 1 0 1 1 1 1 0 0 1 0
0 1 0 1 0 1 0 1 0 0 1 0
10
9
7
6
6
5
water holding capacity did not affect the number of irrigations during the period of the greatest ET variability, which is between October and December. The citrus with active ground cover or weeds required one more irrigations than citrus with no cover (Table 2).
4. Conclusions Useful irrigation guidelines can be developed based on historical data since the coefficient of variability is less than 15% over the period of time and distances studied. For a subtropical, semiarid region such as the LRGV of Texas, a coefficient of 0.7 for the relationship between class-A pan evaporation and Pennman Monteith reference ET is probably most appropriate.
Acknowledgment This research was funded by the USDA under the project ‘‘Efficient Irrigation for Water Conservation in the Rio Grande basin,’’ Project No. 2001-4509-01149.
References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop Evapotranspiration—Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56. Rome. American Meteorological Society, 2000. Glossary of Meteorology. [Online] Available at: //amsglossary.allenpress.com/glossary.
J. Enciso, B. Wiedenfeld / Agricultural Water Management 76 (2005) 1–7
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Bhuiyan, S.I., Undan, R.C., 1990. Irrigation for tropical areas. In: Hoffman, G.J., Howell, T.A., Soloman, K.H. (Eds.), Management of Farm Irrigation Systems. ASAE Monograph No. 9, American Society of Agricultural Engineers, St. Joseph, MO, USA, pp. 581–628. Rozeff, N., 1998. Sugarcane irrigation management. In: Rozeff, N., Amador, J.M., Irvine, J.E. (Eds.), South Texas Sugarcane Production Handbook. TAMU Research and Extension Center, Weslaco, and Rio Grande Valley Sugar Growers, Inc., Santa Rosa, TX. Salinas, F., Namken, L.N., 1977. Irrigation scheduling for sugarcane in the Lower Rio Grande Valley of Texas. Proc. Am. Soc. Sugar Cane Technol. 6, 186–191. Stone, R.C., Hammer, G.L., Marcussen, T., 1996. Prediction of global rainfall probabilities using phases of the southern oscillation index. Nature 384, 52–55. United Nations Educational, Scientific, and Cultural Organization (USNESCO), 1977. World Map of Desertification. U.N. Conference on Desertification. Conference 74/2. FAO. Rome, Italy. Walter, I.A., Allen, R.G., Elliot, R., Jensen, M.E., Itenfisu, D., Mecham, B., Howell, T.A., Snyder, R., Brown, P., Echings, S., Spofford, T., Hattendorf, M., Cuenca, R.H., Wright, J.L., Martin, D., 2000. ASCE’s standardized reference evapotranspiration equation. In: Evans, R.G., Benham, B.L., Trooien, T.P. (Eds.), Proceedings of the Fourth Decennial Symposium, National Irrigation Symposium. Am. Soc. Agric. Eng., St. Joseph, MI, pp. 209–215.
Irrigation guidelines based on historical weather data in the Lower Rio Grande Valley of Texas Juan Enciso, Bob Wiedenfeld * Texas A&M University Research and Extension Center, 2415 E. Highway 83, Weslaco, TX 78596, USA Accepted 18 February 2005 Available online 14 March 2005
Abstract Irrigation guidelines based on historical data would be an important tool for planning irrigation, and may be considered as a management strategy depending on the weather variability. This paper analyzes variability in evapotranspiration (ET) patterns over a 9-year period between years and between different months. ET and rainfall data were collected with automatic weather stations and coefficients of variability were determined. The coefficient of variability for these data averaged less than 15% indicating the possibility of using these irrigation guidelines. The coefficient for the relationship between Pennman Monteith reference ET and class-A pan evaporation was found to be 0.7. Irrigation guidelines for sugarcane and citrus based on historical data were developed for irrigation management. # 2005 Elsevier B.V. All rights reserved. Keywords: Evapotranspiration; Pan evaporation
1. Introduction There are several networks providing weather information in Texas, including the National Weather Service, Mesonet, the Texas ET Network, and the Crop Weather Program for Southwest Texas. The National Weather Service and the Mesonet provide broadly dispersed regional weather data across the state while the Texas ET Network and the Crop Weather Program provide evapotranspiration (ET) data specific for local * Corresponding author. Tel.: +1 9569685585; fax: +1 9569695620. E-mail address: [email protected] (B. Wiedenfeld). 0378-3774/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2005.02.009
2
J. Enciso, B. Wiedenfeld / Agricultural Water Management 76 (2005) 1–7
conditions. The information provided by these networks is reported daily and can be accessed through the internet. Many years of data have been collected by these networks. Farmers are being trained in the use of ET for irrigation management, however, irrigation scheduling based on actual ET data has been adopted slowly by a limited number of growers. In Southwest Texas farmers have preferred the use of irrigation sensors such as capacitance probes and Watermark1 sensors to schedule irrigation. Generally, growers manage their irrigation by determining when to initiate water application as indicated by these sensors, and by making certain assumptions regarding soil water availability and depletion levels in the soil. Farmers give several reasons for why they do not use daily ET including: (1) ET does not take into account crop stress such as diseases; (2) they already know crop water requirements and these are constant year-to-year; (3) they want to irrigate based on direct observation of the condition of the plant; and (4) soil water balance is difficult for their particular soil and the internet is difficult to access. In some locations, general guidelines based on standard crop water requirements and number of irrigations depending on soil type have been adopted. These guidelines indicate the number of irrigations and amounts by crop, and were developed using average historical ET data. Examples include crops such as sugarcane that has recommendations provided in the South Texas Sugarcane Production Handbook (Rozeff, 1998), or citrus producers that have knowledge based on experience regarding seasonal irrigation requirements. The climate of the Lower Rio Grande Valley is identified as semiarid according to criteria established by USNESCO (1977) which identifies climatic zones according to an aridity index (precipitation divided by reference ET). The most variable weather parameter and the most unpredictable is rainfall. The coefficient of variability for rainfall in humid areas has been estimated to be 10 to 20%, whereas in semi-arid areas it may vary from 20 to 30% (Bhuiyan and Undan, 1990). Temperature variability declines as one moves from a temperate to a tropical climate. The Lower Rio Grande Valley is located at 308 N latitude just north of the tropic of cancer (23.58 N latitude), and extends from the coast to 150 km inland from the Gulf of Mexico. The objectives of this study were to evaluate the coefficient of variability in ET over a period of 9 years, to use average ET to develop irrigation guidelines for sugarcane and citrus, and to determine the relationship between ET and class-A pan evaporation in the subtropical, semiarid Lower Rio Grande Valley of Texas.
2. Materials and methods Nine years of weather data were collected from three weather stations in the Lower Rio Grande Valley of Texas. Stations were located on three farms of the Texas A&M Research and Extension Center at Weslaco between 2.5 and 5.3 km from each other. Weather stations were separated by distances comparable to situations where farmers use weather data from stations located in their proximity to estimate ET. Weather stations were Campbell Scientific model Met Data 1 or ET106 containing CR10X data loggers with the following sensors: Vaisala CS500 temperature and relative humidity probe, Met One 034A windset, Li-Cor LI-200X pyranometer, and Texas Instruments TE525 tipping bucket rain gage. Data collected were hourly average and daily maximum and minimum temperature and relative
J. Enciso, B. Wiedenfeld / Agricultural Water Management 76 (2005) 1–7
3
humidity, daily average wind speed, cumulative daily solar radiation and rainfall. ET was calculated daily using the Penman-Monteith ASCE standardized reference equation (Walter et al., 2000). Evaporation from a U.S. Weather Bureau evaporation pan (class-A pan, American Meteorological Society, 2000) was manually recorded daily at one location. Mean and standard deviation were calculated for data totaled by month for each station. When any data was missing, that month was not included in the calculations. Monthly sugarcane and citrus ET was calculated by using crop coefficients recommended by Allen et al. (1998). The sugarcane crop coefficient curve was adjusted to local growing conditions based on sugarcane water use determined by Salinas and Namken (1977). Irrigation guidelines were developed using the Harlingen silty fine clay, Raymondville clay loam and Rio Grand silt loam soils with water holding capacities of 91, 122, and 152 mm per 60 cm of soil depth, respectively, that represent a wide range of irrigation depths.
3. Results Average ET, pan evaporation and rainfall data is presented in Fig. 1. Pan evaporation was always higher than reference ET, and ET higher than rainfall. Highest average monthly rainfall received was during September with 127 mm followed by October with 86 mm, and this is one of the reasons that little irrigation is required from September through December. The wettest part of the year is between August and October in this region. The development of season climate forecasting based on the El Nin˜ o Southern Oscillation (ENSO) phenomenon has introduced the possibility of crop management based on seasonal forecasts (Stone et al., 1996). Management could be adjusted according to probable weather trends such as rainfall. The coefficient of variability for rainfall in this study was highly variable. The average of the lowest coefficient of variability of the three weather stations was 63% for the month of May. Rainfall predictions are beyond the scope
Fig. 1. Average monthly class-A pan evaporation, Penman-Monteith reference ET and rainfall for 1995–2003 in the Lower Rio Grande Valley of Texas.
4
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Fig. 2. Coefficient of variability (CV) for monthly reference ET during a 9-year period at three locations and averaged across location in the Lower Rio Grande Valley of Texas.
of this study, but future research could associate seasonal rainfall with the Southern Oscillation Index (SOI) or Sea Surface Temperatures (SST). The coefficient of variability for reference ET varied between the three stations, and also over time at each station (Fig. 2). The Annex station showed the least variability, while higher coefficients of variability were observed at the Center and Hiler Stations. Over time the coefficient of variability averaged 14% from February to August then increased from September through January but always remained below 25% with the exception of December that was 38% (Fig. 2). This would indicate that irrigation guidelines for the first 8 months are more accurate but then become more variable, probably varying by as much as 30% after September. It is important to note that lower coefficients of variability were observed during the months when ET demand, and therefore, irrigation requirements would be highest. In September when variability starts to increase, ET begins to decline (Fig. 3). This also coincides with the time when farmers in the Lower Rio Grande Valley attempt to deplete soil moisture by limiting irrigation for sugarcane to promote sugar
Fig. 3. Percent of annual reference ET by month, and coefficient of variability (CV) for monthly reference ET for a 9-year period in the Lower Rio Grande Valley of Texas.
J. Enciso, B. Wiedenfeld / Agricultural Water Management 76 (2005) 1–7
5
Fig. 4. Ratio between reference ET and class-A pan evaporation for 1995-2003 at the Texas A&M University Research and Extension Center weather station, Weslaco, TX.
accumulation. The importance of having accurate data after September is reduced because besides declining ET demand, more rain is normally received during this period. Some farmers also like to base their predictions on pan evaporation. Pan evaporation was always higher than reference ET, with the ratio between reference ET and class-A pan ranging between 0.65 and 0.85 (Fig. 4). This indicates that a pan evaporation ratio of 0.7 may be used to estimate ET. Table 1 presents the number of irrigation required for sugarcane for the three types of soils assuming a management allowable depletion of 60% and using average ET and rainfall. Only one irrigation was required during November for sugarcane and it was for the soils with lower water holding capacity. The soil with holding capacity greater than 152 mm did not require irrigation for sugarcane after September. A similar trend was observed with citrus, which required one irrigation during November. However, the soil Table 1 Monthly average reference ET, rainfall, crop coefficients (Kc) and number of irrigations required for sugarcane for three soil types with different root depths in the Lower Rio Grande Valley of Texas Month
January February March April May June July August September October November December Total
ETo (mm)
Rain (mm)
Kc sugarcane
87 95 127 150 181 183 198 190 148 125 97 79
6 9 37 33 34 62 48 63 126 86 45 10
0.40 0.40 0.40 0.64 1.15 1.25 1.25 1.25 1.10 0.70 0.40 0.40
1660
560
Harlingen fine clay (91 mm)
Raymondville clay loam (122 mm)
Rio Grande Silt loam (152 mm)
1 1 1 1 2 2 2 1 0 0 1 0
0 1 1 1 2 1 1 1 0 0 1 0
0 1 1 1 1 1 1 1 0 0 0 0
12
9
7
6
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Table 2 Monthly crop coefficients (Kc) and number of irrigations required for mature citrus trees with 70% canopy cover with and without active ground cover or weeds Month
January February March April May June July August September October November December
Harlingen fine clay
Kc
Raymondville clay loam Rio Grande silt loam
With cover No cover With cover No cover With cover No cover
With cover No cover
0.75 0.75 0.75 0.75 0.70 0.70 0.70 0.70 0.75 0.75 0.75 0.75
Total
0.70 0.70 0.70 0.70 0.65 0.65 0.65 0.65 0.70 0.70 0.70 0.70
0 1 1 1 1 2 2 1 0 0 1 0
1 1 1 1 1 1 1 1 0 0 1 0
0 1 0 1 1 1 1 1 0 0 1 0
0 1 0 1 1 1 0 1 0 0 1 0
0 0 1 0 1 1 1 1 0 0 1 0
0 1 0 1 0 1 0 1 0 0 1 0
10
9
7
6
6
5
water holding capacity did not affect the number of irrigations during the period of the greatest ET variability, which is between October and December. The citrus with active ground cover or weeds required one more irrigations than citrus with no cover (Table 2).
4. Conclusions Useful irrigation guidelines can be developed based on historical data since the coefficient of variability is less than 15% over the period of time and distances studied. For a subtropical, semiarid region such as the LRGV of Texas, a coefficient of 0.7 for the relationship between class-A pan evaporation and Pennman Monteith reference ET is probably most appropriate.
Acknowledgment This research was funded by the USDA under the project ‘‘Efficient Irrigation for Water Conservation in the Rio Grande basin,’’ Project No. 2001-4509-01149.
References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop Evapotranspiration—Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56. Rome. American Meteorological Society, 2000. Glossary of Meteorology. [Online] Available at: //amsglossary.allenpress.com/glossary.
J. Enciso, B. Wiedenfeld / Agricultural Water Management 76 (2005) 1–7
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Bhuiyan, S.I., Undan, R.C., 1990. Irrigation for tropical areas. In: Hoffman, G.J., Howell, T.A., Soloman, K.H. (Eds.), Management of Farm Irrigation Systems. ASAE Monograph No. 9, American Society of Agricultural Engineers, St. Joseph, MO, USA, pp. 581–628. Rozeff, N., 1998. Sugarcane irrigation management. In: Rozeff, N., Amador, J.M., Irvine, J.E. (Eds.), South Texas Sugarcane Production Handbook. TAMU Research and Extension Center, Weslaco, and Rio Grande Valley Sugar Growers, Inc., Santa Rosa, TX. Salinas, F., Namken, L.N., 1977. Irrigation scheduling for sugarcane in the Lower Rio Grande Valley of Texas. Proc. Am. Soc. Sugar Cane Technol. 6, 186–191. Stone, R.C., Hammer, G.L., Marcussen, T., 1996. Prediction of global rainfall probabilities using phases of the southern oscillation index. Nature 384, 52–55. United Nations Educational, Scientific, and Cultural Organization (USNESCO), 1977. World Map of Desertification. U.N. Conference on Desertification. Conference 74/2. FAO. Rome, Italy. Walter, I.A., Allen, R.G., Elliot, R., Jensen, M.E., Itenfisu, D., Mecham, B., Howell, T.A., Snyder, R., Brown, P., Echings, S., Spofford, T., Hattendorf, M., Cuenca, R.H., Wright, J.L., Martin, D., 2000. ASCE’s standardized reference evapotranspiration equation. In: Evans, R.G., Benham, B.L., Trooien, T.P. (Eds.), Proceedings of the Fourth Decennial Symposium, National Irrigation Symposium. Am. Soc. Agric. Eng., St. Joseph, MI, pp. 209–215.