Spiro GRAZHDANI, Alma Ahmeti Agricultural University of Tirana

1. Estimating Reference Evapotranspiration Using Two Different Models of Penman-Monteith Method for Climatic Conditions of Albania Spiro GRAZHDANI, Alma Ahmeti Agricultural University of Tirana

2. It is known now that accurate and consistent determination of ET in irrigated agriculture is becoming increasingly important for better planning and efficient use of water resources. Numerous methods have been introduced for computing ET0 (because direct measurement of ET0 is difficult and costly). Among these methods are selected two as the most correct: FAO 56 PENMAN - MONTEITH ET0 = STANDARDIZED ASCE PENMAN - MONTEITH ET0 =

3. FAO 56 PENMAN-MONTEITH METHOD Equacion ET0 = The experts of FAO selected the FAO 56 P-M method because: • It closely approximates grass ET0 at the location evaluated; • Is physically based; • Explicitly incorporates both physiological and aerodynamic parameters; • Have been developed procedures for estimating missing climatic parameters;

4. STANDARDIZED ASCE PENMAN–MONTEITH METHOD Equacion The constants vary as a function of the reference surface and time step (hourly or daily) ET0 = • Advantages: • Providing commonality in computing ET0; • Enhancing the transferability of crop coefficients;

5. A literature review reveals that ET0 methods are being mainly utilized for computation with a 24 h time step and not as a sum-of-hourly basis. But the sum-of hourly approach should provide a better representation of the effect of climatic conditions (solar radiation, air temperature, wind speed, and vapor pressure deficit) on daily ET0. THE AIM OF THIS STUDY • The comparison of two methods mentioned above; • The comparison of ET0 methods using different time step; The standardized ASCE-PM sum-of-hourly ET0 (ET0,h,ASCE ) computations was used as the basis for comparison of ET0 values because for daytime hourly periods, rs is less than 70 s m-1 for the standardized height of 0.12 m, which is used in FAO 56-PM for clipped grass, and that lower rs values (e.g., 50 s m-1 used in the standardized ASCE-PM method) would better represent clipped grass hourly rs values under the field conditions.

6. The study sites provided an opportunity to compare performance of the ASCE-PM, and FAO 56-PM computation procedures, and hourly and daily time steps, over a relatively wide range of climates and over a range of elevations.

7. COMPARISONS • Comparison of 24 h time step (ET0,d) and sum-of-hourly ASCE-PM ET0 (ET0,h,ASCE); • Comparison of ET0,d and ET0,h,ASCE for peak month; • Comparison of sum-of hourly ASCE-PM (ET0,h,ASCE) and sum-of-hourly FAO 56-PM (ET0,h,FAO) methods; • Multiple days and longer-term comparisons of ET0,d and ET0,h,ASCE for calendar year and peak ET0 month;

8. 1. Comparison of 24 h time step (ET0,d) and sum-of-hourly ASCE-PM ET0 (ET0,h,ASCE) (a) Values in parenthesis indicate standard deviation between ET0,d and ET0,h,ASCE (b) Regression coefficients where ET0,d = a ET0,h, ASCE + b Statistics and results of paired sample t-test (two-sample for means) for the ET0,d versus ET0,h,ASCE (mm d-1) values (α = 0.5) for the calendar year

9. Burreli Tirana Korça Lushnja Gjirokastra Vlora Relationship between ET0,d and ET0,h,ASCE values

10. RESULTS • The relationship was good at all locations; • Burreli and Korça had the lowest RMSD value (0.28 and 0.32 mm d-1) among all sites, and Gjirokastra and Vlora had the highest; • Average ratios of ET0,d to ET0,h,ASCE were close to 1.0 for the Burreli and Tirana stations, ranging from 0.98 to 1.01 with lesser scatter around 1:1 line; • The average ratio of ET0,d to ET0,h,ASCE ranged from 0.97 at Korça to 1.08 at Lushnja, indicating that the ET0,d estimated 3.9% lower than ET0,h,ASCE at Korça and estimated 7.8% higher at Lushnja for the calendar year; • ET0,d values were significantly different (P < 0.5) from the ET0,h,ASCE values. The null hypothesis (thar are equal), was rejected for all locations;

11. 2. Comparison of ET0,d and ET0,h,ASCE for peak month Peak month ET0 statistics between ET0,d and ET0,h,ASCE values • Values in parenthesis indicate standard deviation between ET0,d and ET0,h, ASCE • (b) Regression coefficients where ET0,d = a ET0,h, ASCE + b

12. Burreli Tirana Lushnja Korça Gjirokastra Relationship between ET0,d and ET0,h,ASCE for peak month Vlora

13. RESULTS • The agreement between ET0,d and ET0,h,ASCE for the peak month exhibited variation from one location to another, although ratios were still close to 1.0; • The RMSD value for Burreli, Korça and Tirana were lower (respectively 0.21, 0.23 and 0.24 mm d-1) for peak ET0 month than for entire year (respectively 0.28, 0.32, and 0.33 mm d-1); • ET0,d estimated lower than the ET0,h,ASCE during the calendar year and during the peak month at two of six locations (Burreli,Korça). The ratio of ET0,d to ET0,h,ASCE time step rang from 0.95 to 1.05; • ET0,d was 3.2% higher than ET0,h,ASCE at Vlora, and 5.3% higher at Lushnja during the peak month;

14. 3. Comparison of sum-of hourly ASCE-PM (ET0,h,ASCE) and sum-of-hourly FAO56-PM (ET0,h,FAO) methods Performanceindicators between ET0,h,ASCE and ET0,h,FAO for the calendar year Statistical analyses (paired sample t-test) between ET0,h,ASCE versus ET0,h,FAO (mm d-1) values (α = 0.5) for the calendar year

15. Burreli Tirana Lushnja Korça Vlora Gjirokastra Relationship between ET0,h,FAO dhe ET0,h,ASCE values

16. RESULTS • ET0,h,ASCE uses coefficients representing rs = 50 s m-1 during daytime and rs = 200 s m-1 during nighttime, whereas ET0,h,FAO uses coefficients representing rs = 70 s m-1 for both daytime and nighttime; • There are a good correlation between ET0,h,ASCE and ET0,h,FAO values at all locations with r2 ≥ 0.990; • ET0,h,FAO method estimated lower than standardized ET0,h,ASCE method at all locations and for all years due to the higher daytime rs. Lower estimation is reflected in the average ratio of ET0,h,FAO to ET0,h,ASCE value; • Estimations by the ET0,h,FAO ranged from -4.8 % at Burreli to - 7.8 % at Vlora relative to ET0,h,ASCE;

17. 4. Multiple days and longer-term comparisons of ET0,d and ET0,h,ASCE for calendar year and peak ET0 month Performance indicators between ET0,d and ET0,h,ASCE values for multiple days and longer periods • (a) Values in parenthesis indicate standard deviation between ET0,d and ET0,h, ASCE • Average ratio ET0,d/ET0,h, ASCE • Regression coefficients where ET0,d = a ET0,h, ASCE + b

18. RESULTS This process would help to assess whether summing daily ET0 values over longer periods would reduce the risk of using daily (ET0,d) values as compared with the ET0 values computed on an sum-of-hourly basis. • Summing the ET0 values over a weekly, and monthly basis somewhat reduced the differences between ET0,d and ET0,h,ASCE values as compared with the values reported in the first comparison. For example at Burreli, the r2 increased from 0.984 to 0.996, the average ratio decreased from 0.98 to 0.94 with data points scattering closer to 1:1 line when ET0 values were summed on a weekly period; • The Korça station resulted in the smallest difference (11.7 mm year-1) between the two ET0 computation procedures on an annual basis. The Vlora and Lushnja stations resulted in the largest annual differences 79.5 dhe 62.1 mm year-1, which is a considerable difference in terms of designing and planning of irrigation and drainage systems; • These differences suggest that using a 24 h time step rather than sum-of hourly time step would result in underestimations of ET0 from 5% to 8% based on the weekly, monthly, and annual average ratio. This may cause improper design of water management infrastructure;

19. Performance indicators between ET0,d and ET0,h,ASCE values for multiple days for peak ET0 month (a) Values in parenthesis indicate standard deviation between ET0,d and ET0,h, ASCE (b) Average ratio ET0,d/ET0,h, ASCE (c)Regression coefficients where ET0,d = a ET0,h, ASCE + b

20. RESULTS • The difference were lower (higher r2 between ET0,d and ET0,h,ASCE values) at Burreli, Tirana and Korça, whereas the differences were higher (lower r2 values, higher deviation) at Gjirokastra, Vlora, and Lushnja; • These results suggest that summing the ET0,d values over multiple days and longer periods for the peak ET0 month resulted in inconsistent differences with locations; • Summing the ET0,d values over multiple days improved (lower RMSD and higher r2), the relationship between the two ET0 computation procedures at Tirana, Burreli, and Korça for the peak ET0 month;

21. CONCLUSIONS • Comparisons between the ET0,d versus ET0,h,ASCE values indicated that there are significant differences between two sets of ET0 values, attributed partly to the inability of the 24 h time step computation procedure to account for the effect of abnormal diurnal changes of climatic conditions and partly to the use of higher surface resistance (70 s m-1) during daytime periods in the hourly time step application; • The differences suggested that using a 24 h time step rather than sum-of-hourly approach would result in underestimations of ET0 of as much as 5% to 8% depending on the location; • Summing the ET0 values over a weekly, monthly, and annual basis (for the calendar year) somewhat reduced the differences between the ET0,d and ET0,h,ASCE values, but not with similar magnitudes at all locations; • Results suggest the benefit and potentially improved accuracy of using the standardized ASCE-PM procedure applied hourly as opposed to applying it with a 24 h time step basis. The hourly application helps to account for impacts of abrupt diurnal changes in atmospheric conditions on ET0 estimation in advective and other environments, when hourly climate data are available.

22. THANK YOU ! Balwois 2010 - Ohrid, May 2010