Invention Journal of Research Technology in Engineering & Management (IJRTEM) ISSN: 2455-3689 www.Ijrtem. com Volume 3 Issue 5 ǁ July –August 2019 ǁ PP 06-12
Estimation of Evapotranspiration using CROPWAT 8.0 Model for Taung-Nyo Dam Cultivated Area, Myanmar Mi Yin San1, May Thinzar2 1
(Civil Department, West Yangon Technological University, Myanmar) 2 (Civil Department, Pyay Technological University, Myanmar)
ABSTRACT : The estimate of actual evapotranspiration is very important not only in the study of climate changes and evaluation of water resources but also in crop water requirement scheduling etc. This study was conducted to estimate the evapotranspiration using CROPWAT 8.0 Model for Taung-Nyo dam cultivated area. It was collected the long term daily meteorological data including maximum temperature, minimum temperature, relative humidity, wind speed and sunshine hours from the Meteorological and Hydrological Department to calculate the reference evapotranspiration, ETO. Crop coefficient, KC was multiplied with ETO which is a measure of evaporative demand to get actual evapotranspiration, ET. The major crops in this cultivated area are Monsoon season crops which are paddy, sesame and maize and dry season crops which are green gram, bean, sugarcane and groundnut. The study observed that the maximum actual ET are 170.6 mm and 186.8 mm in the month of April for the average period and for each year, respectively due to high temperature and the minimum are 89.093 mm and 85.513 mm in December. So, it can be concluded from the observation that as the temperature increases, the ET increases and as the temperature decreases, it also decreases.
KEYWORDS: reference evapotranspiration, actual evapotranspiration, CROPWAT, crop coefficients, meteorological parameter
I.
INTRODUCTION
Actual evapotranspiration is defined as the rate of evapotranspiration by particular crop in a given period under prevailing soil water and atmosphere condition. The CROPWAT is an irrigation management and planning model simulating the complex relationships of on-farm parameters, the climate, soil and crop.In arid regions, agricultural development strongly depends on the irrigation because of low rainfall conditions and high evaporation. The transfer of water from the soil surface (evaporation) and plants (transpiration) to the atmosphere is referred to evapotranspiration. From the irrigation point of view, the ET estimates the amount of water to be applied through the artificial means. Using the ET, the sizes of canals and pumps are determined. Many different methods are used to find the ETO. These methods are Blaney-Criddle method, Radiation method, Pan evaporation method, Penman method or Modified Penman method and FAO Penman-Monteith method, etc. Hashem et al., (2016) made an attempt to compare the reference evapotranspiration computed using a mathematical model with ETO estimated using CROPWAT software program. Vivekanand Singh et al., (2006) calculated ETO by using seven different methods and as a standard Penman-Monteith method was used and compared with the other six methods. It is found to be good correlation between them. In this study, the ETO is calculated using the CROPWAT 8.0 model and its value is multiplied with crop coefficient to get the actual evapotranspiration. In the fields of irrigation and water management, estimates of reference ET are used to make irrigation recommendations for growers. Thus, the current study conducted at Taung-Nyo cultivated area can be used as a reference for calculating ET in other area having the same conditions. This study provides the necessary information on water requirements for growing different crops in different season.
II.
MATERIALS AND METHODS
Study Area : Taung-Nyo dam is situated near Magyibin village, Nattlin Township in Bago division. It is located at north latitude: 18.5167º, east Longitude: 95.60 ° and its altitude is 82 m above mean sea level. Taug-Nyo dam project is proposed to irrigate 50000 acres in project area. The study area comprises of Zigon Township and Nattlin Township. The location of the project area is shown in Fig 1.
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Estimation of Evapotranspiration using CROPWAT 8.0 Model…
Fig. 1 Map showing the location of the cultivated area Source: Ministry of Agriculture and Irrigation Department Available of Hydrological Data : In the meteorological data collection, it requires temperature, humidity, and wind speed and shine hours to calculate the ETO. Almost these data from 2001 to 2010 are obtained from Department of Meteorology and Weather Forecast, Pyay. Temperature is recorded for both maximum and minimum temperature. CROPWAT 8.0: CROPWAT is a decision support tool developed by the land and Water Development Division of FAO. CROPWAT 8.0 for Windows is a computer program for the calculation of crop water requirements and irrigation requirements based on soil, climate and crop data. In addition, the program allows the development of irrigation schedules for different management conditions and the calculation of scheme water supply for varying crop patterns. CROPWAT 8.0 can also be used to evaluate farmers’ irrigation practices and to estimate crop performance under both rained and irrigated conditions. The development of irrigations schedules in CROPWAT 8.0 is based on a daily soil-water balance using various user-defined options for water supply and irrigation management conditions. Data required in estimation of potential evapotranspiration The input data used for calculating the potential evapotranspiration are listed below• • • • • •
Minimum temperature (°C) Maximum temperature (°C) Sunshine hours (hrs) Wind speed (km/day) Relative humidity (%) Latitude Longitude Altitude
Reference of Evapotranspiration : From the original Penman-Monteith equation and the equations of the aerodynamic and surface resistance, the FAO Penman-Monteith method can be derived to estimate ETO. For computation of daily reference of evapotranspiration, the Penman-Monteith equation assumes the reference crop evapotranspiration as that from a hypothetical crop with assumed height of 8 to 15 cm tall, green grass cover of uniform height, actively growing, completely shading to the ground and adequately water. It is expressed as
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Estimation of Evapotranspiration using CROPWAT 8.0 Model‌ đ??¸đ?‘‡đ?‘œ = Where, ETO = Rn = G = T = u2 =
900 đ?‘˘ (đ?‘’ −đ?‘’ ) đ?‘‡+273 2 đ?‘ đ?‘Ž
0.408∆(đ?‘…đ?‘ −đ??ş)+Ď’
(1)
∆+Ď’ (1+0.34đ?‘˘2 )
reference evapotranspiration (mm day-1) net radiation at the crop surface (MJ m-2 day-1) soil heat flux density (MJ m-2 day-1) main daily air temperature at 2m height (ÂşC) wind speed at 2m height (ms-1) 4.87 u2= đ?‘˘đ?‘§ ln( 67.8 đ?‘§âˆ’5.42)
es = saturation vapor pressure (kPa) ea = actual vapor pressure (kPa) es- ea = saturation vapor pressure deficit (kPa) ∆ = slope vapor pressure curve (kPa ÂşC-1) Ď’ = psychrometric constant (kPa ÂşC-1) It takes long time to get ETO by this method without using software. The CROPWAT model, was developed and recommended by the Department of Land and Water Resources of FAO, is a computer program for the calculation of crop water demand requirements using Penman-Monteith equation based on soil, climate and crop data. Actual Evapotranspiration : Actual evapotranspiration is obtained by multiplying ETO resulted from the CROPWAT model with the crop coefficient (đ??žđ??ś ) which accounts for crop characteristics and management practices (i.e, frequency of wetness). đ??¸đ?‘‡ = đ??žđ??ś Ă— đ??¸đ?‘‡đ?‘œ (2) The value of reference crop evapotranspiration is estimated by using different methods based on climatological parameters. The crop coefficient (Kc) varies according to the growing states. Only three values for Kc are required during the initial stage, the mid-season stage and the end of the late season stage. Typical values for K c ini, Kc mid and Kc end for all proposed crop types which may be used in Cropwat calculation are given in Table-I. The average crop coefficient for each month is represented in Table-3. TABLE 1. CROP COEFFICIENT VALUES Name of crops Paddy
Dry Wet
Sesame Maize Green gram Bean Sugarcane Groundnut
Kc ini 0.5 1.1 0.35 0.7 0.4 0.5 0.4 0.4
Kc mid 1.05 1.2 1.1 1.2 1.05 1.05 1.25 1.15
Kc end 0.7 1.05 0.25 0.6 0.6 0.9 0.75 0.6
TABLE 2. CROP COEFFICIENT VALUES USED IN PROJECT AREA Growth Stage
Rice
Growth Stage
Black Gram
0-60 days after transplant Mid-season Last 30 days before harvest
1.1 1.2 1
Mid season Late season
1.15 1.05
Source: FAO, Irrigation Water Management Training Manual No.3
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Estimation of Evapotranspiration using CROPWAT 8.0 Model… TABLE 3. AVERAGE CROP COEFFICIENT FOR EACH MONTH Month January February
Avg: Kc 1.15 1.05
Year July August
Avg: Kc 1.1 1.05
March
1.1
September
1.03
April
1.2
October
1.00
May
1.0
November
1.15
June
1.1
December
1.05
TABLE 4. ETO(MM/DAY) BY PENMAN-MONTEITH EQUATION Year/Month 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Average
Jan 2.95 2.89 3.08 3.09 3.00 3.06 3.04 3.07 3.19 2.92 3.12
Feb 3.54 3.52 3.63 3.47 3.57 3.57 3.36 3.50 3.57 3.56 3.54
Mar 4.31 4.18 4.38 4.19 4.15 4.38 4.28 4.01 3.53 3.67 4.23
Apr 4.65 4.63 4.88 4.64 4.65 5.19 4.80 4.80 5.04 4.87 4.74
May 4.12 4.98 4.44 4.03 4.01 3.96 3.81 3.80 4.69 5.60 4.34
Jun 3.71 3.81 3.87 3.82 3.31 3.79 4.03 3.33 3.06 3.30 3.60
Jul 3.16 3.23 3.19 3.23 3.38 3.21 3.20 2.89 3.17 3.77 3.24
Aug 3.43 3.36 3.61 3.60 3.45 3.38 3.65 3.39 3.30 3.56 3.46
Sep 3.50 3.52 3.71 3.54 3.62 3.80 3.49 3.58 3.58 3.91 3.62
Oct 4.38 3.85 3.81 3.76 3.82 3.77 3.64 3.61 4.01 3.16 3.78
Nov 3.59 3.42 3.31 3.42 3.48 3.52 3.00 3.25 3.75 3.59 3.42
Dec 3.08 3.07 2.43 2.99 3.08 2.93 2.94 2.94 2.76 2.75 2.86
TABLE 5. ETO(MM/MONTH) BY PENMAN-MONTEITH EQUATION Year/Month 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Average
Jan 91.45 89.59 95.48 95.79 93.00 94.86 94.24 95.17 98.89 90.52 96.72
Feb 99.12 98.56 101.64 97.16 99.96 99.96 94.08 98 99.96 99.68 99.12
Mar 133.61 129.58 135.78 129.89 131.75 135.78 132.68 124.31 109.43 113.77 131.13
Apr 139.5 138.9 146.4 139.2 139.5 155.7 144 144 151.2 146.1 142.2
May 127.72 154.38 137.64 124.93 124.31 122.76 118.11 117.8 145.39 173.6 134.54
Jun 111.3 114.3 116.1 114.6 99.3 113.7 120.9 99.9 91.8 99 108
Jul 97.96 100.13 98.89 100.13 104.78 99.51 99.2 89.59 98.27 116.87 100.44
Aug 106.33 104.16 111.91 111.6 106.95 104.78 113.15 105.09 102.3 110.36 107.26
Sep 105 105.6 111.3 106.2 108.6 114 104.7 107.4 107.4 117.3 108.6
Oct 135.78 119.35 118.11 116.56 118.42 116.87 112.84 111.91 124.31 97.96 117.18
Nov 107.7 102.6 99.3 102.6 104.4 105.6 90 97.5 112.5 107.7 102.6
Dec 95.48 95.17 85.33 92.69 95.48 90.83 91.14 91.14 85.56 85.25 88.66
TABLE 6. ET(MM/MONTH) BY PENMAN-MONTEITH EQUATION Year/Month 2001 2002 2003 2004 2005 2006 2007 2008 2009
Jan 105.1 103.0 109.8 110.1 106.9 109.0 108.3 109.4 113.7
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Feb 104.0 103.4 106.7 102.0 104.9 104.9 98.7 102.9 104.9
Mar 146.9 142.5 149.3 142.8 144.9 149.3 145.9 136.7 120.3
Apr 167.4 166.6 175.6 167.0 167.4 186.8 172.8 172.8 181.4
May 127.7 154.3 137.6 124.9 124.3 122.7 118.1 117.8 145.3
Jun 122.4 125.7 127.7 126.0 109.2 125.0 132.9 109.8 100.9
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Jul 107.7 110.1 108.7 110.1 115.2 109.4 109.1 98.5 108.1
Aug 111.6 109.3 117.5 117.1 112.3 110.0 118.8 110.3 107.4
Sep 108.1 108.7 114.6 109.3 111.8 117.4 107.8 110.6 110.6
Oct 135.7 119.3 118.1 116.5 118.4 116.8 112.8 111.9 124.3
Nov 123.8 117.9 114.2 117.9 120.0 121.4 103.5 112.1 129.3
Dec 100.25 99.929 89.597 97.325 100.25 95.372 95.697 95.697 89.838
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Estimation of Evapotranspiration using CROPWAT 8.0 Model… 2010 Average
104.1 111.2
104.6 104.0
125.1 144.2
175.3 170.6
173.6 134.5
108.9 118.8
128.5 110.4
115.8 112.6
120.8 111.8
97.9 117.1
123.8 117.9
89.513 93.093
TABLE 7. ESTIMATION OF AVERAGE REFERENCE EVAPOTRANSPIRATION FOR THE YEAR 2001TO 2010 (MM/DAY) BY PENMAN-MONTEITH EQUATION Country-Myanmar Altitude-82m Month Min-Tem: Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
ºC 15.82 16.91 20.74 24.21 24.21 24.29 24.25 24.23 24.19 24.87 20.95 17.97
Max Tem
Humid
Wind
Station-Taung Nyo Latitude-18.4ºN Longitude-95.6ºE Sun shine Rad ETo
ºC 35.57 36.23 37.62 39.75 36.79 33.45 32 32.23 32.99 34.3 33.94 32.1
% 70 66 65 66 76 86 87 87 86 85 76 72
Km/d 9 8 9 10 11 11 11 10 9 9 8 4
Hour 8.8 8.9 8.2 8 6.3 4.2 3.3 4.3 5.5 7 8.4 8.5
MJ/m2/d 18.1 20 20.9 21.7 19.3 16.1 14.7 16 17.1 17.8 17.9 17
Mm/day 3.12 3.54 4.23 4.74 4.34 3.6 3.24 3.46 3.62 3.78 3.42 2.86
Annual 1400 1350 1300 Annual
1250
2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
1200
Figure 2. Annual Variation of ETO (mm/month) from 2001 to 2010
Annual 1500.00 1450.00 1400.00 Annual
1350.00
2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
1300.00
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Estimation of Evapotranspiration using CROPWAT 8.0 Model‌ Figure 3. Annual Variation of ET (mm/month) from 2001 to 2010 45 40 35 30 25 20 15 10 5 0
maxËšC min
Figure 4. Mean Temperature Variation from 2001 to 2010
III.
RESULTS AND DISCUSSIONS
In the study, the reference evapotranspiration was calculated using the CROPWAT 8.0 Model and subsequently actual evapotranspiration was obtained by multiplying the reference evapotranspiration with average crop coefficient for the Taung-Nyo cultivated area. The CROPWAT 8.0 Model requires the time series data of minimum temperature, maximum temperature, latitude, longitude, altitude, sunshine hours and wind velocity. The data starting from 2001 to 2010 was collected and analysed for the selected township that is Zigon and Nattlin townships which lie in the Taung-Nyo dam cultivated area. The daily reference evapotranspiration (mm day-1) obtained by CROPWAT 8.0 Model is shown in Table-4. It can clearly be depicted from the Table -5 that the values of maximum monthly ETO (mm month-1) for the average period and for each year are estimated 142.2 mm and 155.7 mm in the month of April, respectively. Whereas monthly ETO for both cases are found as the minimum values in December due to the winter season and low temperature and their respective values are 88.66 mm and 85.25mm. Annual variation of reference evapotranspiration from 2001 to 2010 is described in Figure (2). The average highest annual ETO value is also 1358.11mm due to high temperature during the year 2010 and the lowest is 1281mm in the year 2008. The results of monthly actual evapotranspiration calculated from equation (2) using the CROPWAT are shown in Table 6. It can clearly be depicted from the Table -6 that the values of maximum actual monthly ET are 170.64 mm and 186.84 mm in the month of April for the average period and for each year, respectively. Whereas in December due to the winter season and low temperature, their respective values of minimum actual monthly ET for both cases are 89.093 mm and 85.513 mm. Annual variation of actual evapotranspiration from 2001 to 2010 is described in Figure (3). The average highest annual ET value is also1468.31mm due to high temperature during the year 2010 and the lowest is 1388.82 mm in the year 2008.
IV.
CONCLUSIONS
The Penman-Monteith equation (CROPWAT 8.0 model) is the standard tool for irrigation planning and management because of considering many parameters. In this study, from the meteorological data for 10 years, the ETO for daily is determined using CROPWAT 8.0 model and consequently ETO and ET, for monthly and annually are calculated and compare the results. It can be concluded from the observation that as the temperature increases, the ETO increases and as the temperature decreases, it also decreases. It provides the necessary information on water requirements for growing different crops in different seasons. Using the maximum ET O, net irrigation requirements essential for the canal design can be continued to study.
ACKNOWLEDGEMENT The author is grateful to Dr. Nilar Aye, Professor and Head, Department of Civil Engineering, MTU and Dr. Khin Latt, Professor and Head, Department of Civil Engineering, WYTU for their encouragement and valuable guidance and all teachers from the department of Civil Engineering, WYTU for their useful advices. The author would like to express special thanks to all staff from PTU and Meteorological and Hydrological Department (Pyay) for sharing their experiences.
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Estimation of Evapotranspiration using CROPWAT 8.0 Model‌ REFERENCES [1] [2] [3] [4] [5]
A.M, Michael. 1978. Irrigation Theory and Practice. Vikas Publishing House, Ltd FAO. 1998. Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements, Irrigation and Drainage Paper 56. Food and Agricultural Organization of the United Nation. Smith, M. 2006. Example of the Use of CROPWAT 8.0. Food and Agriculture Organization of the United States. Hashem et al., (2016), performance Evaluation and Development of Daily Reference Evapotranspiration Model, Irrigation Drainage System Eng 2016 Allen et al., (1998) Crop evapotranspiration – guidelines for computing crop water requirements. FAO Irrigation and drainage paper 56. Food and Agriculture Organization, Rome.
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