DETERMINING WATER-YIELD RELATIONSHIP, WATER USE EFFICIENCY, SEASONAL CROP AND PAN COEFFICIENTS FOR ALFALFA IN A SEMIARID REGION WITH HIGH ALTITUDE

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1 482 Bulgarian Journal of Agricultural Science, 16 (No 4) 21, Agricultural Academy DETERMINING WATER-YIELD RELATIONSHIP, WATER USE EFFICIENCY, SEASONAL CROP AND PAN COEFFICIENTS FOR ALFALFA IN A SEMIARID REGION WITH HIGH ALTITUDE Y. KUSLU*, U. SAHIN, T. TUNC and F. M. KIZILOGLU Ataturk University, Faculty of Agriculture, Department of Agricultural Structures and Irrigation, 2524 Erzurum, Turkey Abstract KUSLU, Y., U. SAHIN, T. TUNC and F. M. KIZILOGLU, 21. Determining water-yield relationship, water use efficiency, seasonal crop and pan coefficients for alfalfa in a semiarid region with high altitude. Bulg. J. Agric. Sci., 16: A field study was conducted to determine effects of seasonal deficit irrigation on plant dry forage yield and water use efficiency of alfalfa for a 2-year period in the semiarid region with high altitude. In addition, the seasonal crop and pan coefficients k c and k p of alfalfa was determined in full irrigation conditions. Irrigations were applied when approximately 5% of the usable soil moisture was consumed in the effective rooting depth at the full irrigation treatment. In deficit irrigation treatments, irrigations were applied at the rates of 8, 6, 4, 2 and % of full irrigation treatment on the same day. Irrigation water was applied by hose-drawn traveler with a line of sprinklers. Seasonal actual evapotranspiration, total dry forage yield and water use efficiency was the highest in full irrigation conditions and the lowest in continuous stress conditions. The linear relationship between seasonal actual evapotranspiration and total dry forage yield was obtained. According to the averaged values of 2 years, yield response factor (k y ) was Both seasonal k p and k c were determined as.86 for alfalfa, when combined values of 2 years. Key words: Alfalfa; crop coefficient; pan coefficient; water deficit; yield response factor Abbreviations: - reference evapotranspirasyon (mm); k y - seasonal yield response factor k c - seasonal crop coefficient; k p - seasonal pan coefficient; ET a - actual evapotranspiration, mm; I - the amount of irrigation water applied (mm); P precipitation, mm; C r - capillary rise (mm); D w - amount of drainage water (mm); R f - amount of runoff (mm); Δ S - change in the soil moisture content (mm); Y a - actual harvested yield; Y m - maximum harvested yield, ET m - maximum evapotranspiration, mm; Y d - relative yield reduction; ET d - relative evapotranspiration reduction, mm; E pan - evaporation of Class A pan, mm; R n - net radiation at the crop surface, MJ m -2 day -1 ; G - soil heat flux density, MJ m -2 day -1 ; T - mean daily air temperature at 2 m height, C; u 2 - wind speed at 2 m height, m s -1 ; e s - saturation vapour pressure, kpa; e a - actual vapour pressure, kpa; e s - e a - saturation vapour pressure deficit, kpa; Δ - slope of the saturation vapour pressure curve, kpa/ C; γ - psychrometric constant, kpa/ C; WUE - water use efficiency, kg ha -1 mm -1 ykuslu@atauni.edu.tr or yaseminkuslu@hotmail.com

2 Determining Water-yield Relationship, Water Use Efficiency, Seasonal Crop and Pan Coefficients Introduction Alfalfa has a high water requirement compared to other crops. Evapotranspiration is between 8 and 16 mm growing period depending on climate and length of growing period (FAO, 22a). Seasonal evapotranspiration of alfalfa reported by Sahin and Hanay (1996) was approximately 7 mm in the Erzurum region. In addition, Evren and Sevim (1998) determined evapotranspiration of alfalfa as 678 mm in Erzurum plain conditions. In Erzurum region with semiarid climate, summers are short, hot and dry. Water is the main factor limiting yield production in the hot and dry summer period of semiarid regions. The great challenge for coming decades in arid and semiarid regions will be focusing on increase food production by using less water (FAO, 22b). Therefore, studies are needed to increase the efficiency of the water use that is available. One approach is the development of new irrigation scheduling techniques such as deficit irrigation (Bekele and Tilahun, 27). Deficit irrigation has potential benefits such as increasing irrigation efficiency, decreasing the cost of irrigation and water opportunity cost (English and Raja, 1996). The lack of water in plant and resulting water stress has an important effect on water consumption and yield. Alfalfa is very responsive to water stress (Guitjens, 1993; Orloff et al., 24; Frate and Roberts, 26). Various researchers (Smeal et al., 1991; Grimes et al., 1992; Saeed and El-Nadi, 1997; Bai and Li, 23) recognized a positive linear relationship between alfalfa yield and evapotranspiration. Doorenbos and Kassam (1979) developed a relation between water applied and yield that can measure yield per applied water unit which can largely be practiced with a certain mistake level and can be performed in the conditions of sufficient and deficit water resource. On the other hand, an understanding of water use efficiency was essential for evaluating the field crops in semiarid regions where irrigation water was a limiting factor (Johnson and Henderson, 22). Several alfalfa water use efficiency studies have been reported. Mean annual values fluctuate between 1 and 23 kg ha -1 mm -1 (Grimes et al., 1992; Saeed and El-Nadi, 1997; Hirth et al., 21). The reference evapotranspirasyon ( ) is a measurement of the water use for that reference crop. In the case of, grass is used as the reference; however, other crops may not use the same amount of water as grass due to changes in rooting depth, crop growth stages and plant physiology. The crop coefficient takes into account the crop type and crop development to adjust the for that specific crop. The FAO Penman-Monteith equation is recommended for determining (Allen et al., 1998; Cuiping, 25). Class A pan data is also used extensively throughout the world to estimate (Grismer et al., 22; Irmak et al., 22). However, reliable estimation of using pan evaporation depends on the accurate determination of pan coefficients. Alfalfa is the most important forage crop in Erzurum, in Turkey. Erzurum is accepted as one of the most important animal breeding regions of Turkey. According to last statistics given by Turkish Statistical Institute the alfalfa growing area in the Erzurum region is 3748 ha, and the green, hay and seed production are , and 143 ton, respectively. However, the studies with water consumption of alfalfa plants in the region are not sufficient. Predicting yield response to water use of alfalfa is important in developing strategies and decision-making for use by farmers and their advisors, and researchers for irrigation management under limited water conditions. In semiarid climatic regions with high altitude, however, little attempt has been made to assess the water yield relationships and optimum water management programs for alfalfa. The objectives of this research are: (1) to determine the effect of seasonal water deficit treatments on the plant dry forage yield and water use efficiency of alfalfa in the semiarid conditions with high altitude, (2) to determine the seasonal yield response factor (k y ) that would contribute to irrigation scheduling programs in deficit irrigation treatments for semiarid climatic zones with high altitude, (3) to determine pan and crop

3 484 Y. Kuslu, U. Sahin, T. Tunc and F. M. Kiziloglu coefficients k p and k c enabling the estimation of actual evapotranspiration of alfalfa grown under semiarid conditions with high altitude from Class A pan evaporation. Material and Methods This study was conducted during the growing periods (April-September) of 25 and 26, at the Agricultural Research Station of Ataturk University in Erzurum, in Turkey. The coordinates of the experimental area are 39 º 55 ' N and 41 º 16 ' E. Its altitude is 1835 m from the sea level. The experimental region has a semiarid climate. The climatic variables for growing period of experimental years are given in Table 1. Temperature, humidity, wind speed and sunshine data were taken from Erzurum meteorological station (39 º 57 ' N, 41 º 11 ' E, 1757 m a.s.l., 3 km distance than experimental field). Precipitation and evaporation data were measured with a standard pluviometer and a Class A pan, respectively, located in the experimental area. The pan was placed in a fallow area. Pan readings were taken by daily at the morning time. The soil was classified as an Aridisol according to the US Soil Taxonomy (Soil Survey Staff, 1992) with parent materials mostly consisting of volcanic, marine and lacustrine transported materials. Before establishment of the experiment, soil samples were taken with an auger from the soil layers of -3, 3-6 and 6-9 cm. Some physical and chemical properties of the experimental field soil were determined according to the methods used Klute (1986) and Page et al. (1982) and they were given in Table 2. Table 1 Climatic data of the experimental area in the growing periods of 25 and 26 Years Months Mean maximum temperature, ºC Mean minimum temperature, ºC Mean daily sunshine, h Total precipitation, mm Total evaporation, mm 25 April May June July August September April May June July August September Calculated from the data between 2 and 3 April 2 Calculated from the data between 1and 15 September 3 Calculated from the data between 1and 14 September Climatic parameters Mean relative humidity, % Mean wind speed, m s -1

4 Determining Water-yield Relationship, Water Use Efficiency, Seasonal Crop and Pan Coefficients Table 2 Some physical and chemical properties of the experimental field soil Properties Soil layer, cm Texture Loam Clay loam Clay loam Clay, % Silt, % Sand, % Field capacity, Pw Wilting point, Pw Bulk density, g cm ph* Electrical conductivity, ds m -1 * Carbonates, % Organic matter, % *: in saturated soil extract Fig. 1. The design of experimental field

5 486 Y. Kuslu, U. Sahin, T. Tunc and F. M. Kiziloglu The experiment was laid out as six main plots with an area of 15 m 2 (5 x 3 m) representing five treatments and the control (Figure 1). Each treatment and the control have three replicated sub-plots with an area of 5 m 2 (5 x 1 m). There was a 5 m space between main plots in order to minimize water movement among treatments. Alfalfa (Medicago sativa L. cv. Prosementi) seeds were sown at a rate of 2 kg ha -1 on 2 April 25. All plots received at the same amount of P 2 O 5 (12 kg ha -1 ) and N (5 kg ha -1 ) fertilizer during the sowing and P 2 O 5 (15 kg ha -1 ) fertilizer after the third harvest in 25 year (Serin and Tan, 21). No pesticides were applied. Soil moisture contents in all plots were increased to the field capacity after the sowing in 25 and April 2 in 26. The total usable soil water content within the top.9 m of the soil profile was 1 mm. Irrigations were applied when approximately 5% of the usable soil moisture was consumed in the effective rooting depth (.9 m) at the full irrigation (control) treatment (T-1) in both years (Allen et al., 1998). In deficit irrigation treatments, T-8, T-6, T- 4, T-2 and T- (non-irrigation) irrigations were applied at the rates of 8, 6, 4, 2 and % of full irrigation treatments (T-1) on the same day, respectively. Irrigations were started on June 25 in 25 and May 24 in 26. Irrigation water was applied by the hose-drawn traveler with a line of sprinklers (Figure 1). The total length of sprinkler line is 5 m. The sprinkler line was mounted to a trolley from the central point. During the irrigation water application, the trolley moves toward the irrigation machine. Total 4 sprinkler heads with a nozzle diameter of 8 mm and equal flow rate have been mounted on the sprinkler line with an interval of 1.25 m. The total flow rate of the system is 73.4 m 3 h -1 at the operational pressure of 2 atm. The duration of irrigation water application for each plot has been determined according to the required water amount to be applied to each plot. In this situation, the duration of irrigation water application for T-1, T-8, T-6, T-4 and T-2 plots was 1.2,.82,.61,.41 and.2 h, respectively. Surface flow in plots has not been observed during the water application. In order to obtain irrigation uniformity, irrigation was performed in hours when no wind or low wind (<2 m s -1 ) occurred. Irrigation water was of high quality with an electrical conductivity of.29 ds/m, a sodium adsorption ratio of.41, and a ph of Cuttings were made by machine. Three cuttings were applied in both years. First and second cuttings were made when alfalfa was 1% blooming at the T- 1 treatment plots in both years. Third cuttings were made before blooming because the insufficient climatic conditions. Cutting dates were 3 June, 27 August and 15 September in 25 and 1 June, 1 August and 14 September in 26. Sub-plots were divided into 6 parts each of which is 83.3 m 2 areas. Plant samples cut from the centre of each part were weighed and then dried in an oven at 78 Cº for 24 h before being re-weighed. Yields were then expressed on an oven-dry weight basis. Actual evapotranspiration (ET a ) was calculated as a daily average for 1-day periods during the growing season by the soil water balance equation (Malek and Bingham, 1993; Rana and Katerji, 2; Zhao et al., 24): ET a = I + P+C r - D w R f ± Δ where ET a is the actual evapotranspiration (mm), I is the amount of irrigation water applied (mm), P is the precipitation (mm), C r is the capillary rise (mm), D w is the amount of drainage water (mm), R f is the amount of runoff (mm), and Δ S is the change in the soil moisture content (mm). Change in the soil moisture content at effective rooting depth was measured on days 1, 2 and 3 or 31 of each month, and at all harvests. Furthermore, additional measurements were applied for determining the exact date of irrigation based upon reduction in soil moisture content. In the soil moisture content measurements were used the TDR (TDR 3, spectrum technologies, USA) in top soil layers (-3 and 3-6 cm) and the conventional oven-dry method in below soil layers (6-9 and 9-12 cm). No runoff was observed during the experiments. Capillary rise was considered as negligible S

6 Determining Water-yield Relationship, Water Use Efficiency, Seasonal Crop and Pan Coefficients where Y a is actual harvested yield, Y m is maximum harvested yield, k y is yield response factor, ET a is actual evapotranspiration, ET m is maximum evapotranspiration, Y d is relative yield reduction, and ET d is relative evapotranspiration reduction. Reference evapotranspiration ( ) was calculated as a daily average for 1-day periods during the growing season by using the CropWat for Windows (Version 4.2) program (Clarke et al., 1998). This program uses the FAO Penman-Monteith equation for calculating. The FAO Penman-Monteith equa- because of the deep water table level. Drainage water consist of precipitation under the effective rooting depth, according to the soil water content measurements in soil layer at under the effective rooting depth, was determined. Water use efficiency (kg ha -1 mm -1 ) was calculated by dividing plant dry matter yield (kg ha -1 ) to actual evapotranspiration (mm) (Howell et al., 1992; Scott, 2). The relationship between relative evapotranspiration reduction ( ETa) and relative yield reduction 1 ETm Ya ( 1 Y ) was determined using the method m given by Doorenbos and Kassam (1979). The equations are as follows. Ya ETa 1 = k Y y ( 1 ) m ETm or Y d = k y ET d tion is given by Allen et al. (1998): where is the reference evapotranspiration (mm day -1 ), R n is net radiation at the crop surface (MJ m -2 day -1 ), G is soil heat flux density (MJ m -2 day -1 ), T is mean daily air temperature at 2 m height ( C), u 2 is wind speed at 2 m height (m s -1 ), e s is saturation vapour pressure (kpa), e a is actual vapour pressure (kpa), e s - e a is saturation vapour pressure deficit (kpa), Δ is slope of the saturation vapour pressure curve (kpa/ C), and γ is psychrometric constant (kpa/ C). The crop coefficient (k c ) is the ratio of ET a to, and, similarly, the pan coefficient (k p ) is the ratio of to E pan. k c and k p were estimated with the following equations (Allen et al., 1998; Sentelhas and Folegatti, 23, Snyder et al., 25; Sahin et al., 27): k c = ET a / k p = /E pan Regression technique was used to determine evapotranspiration-dry forage yield relationship, crop and pan coefficient. Analysis of variance (General Linear Model-ANOVA) was conducted to evaluate the effects of treatments on the dry forage yield using MINITAB statistical package (release 11.12, 1996; Minitab Inc.). Duncan s multiple range test was used to compare and rank the treatment means (MSTAT- C, 1988). Results and Discussion Water-Yield Relationship The total number of irrigation, the amount of irrigation water and actual evapotranspiration (ET a ) values of alfalfa for the experimental years was presented in Table 3. The total number of irrigations, the applied irrigation water and the seasonal ET a in 25 growing period were lower than in 26. This may be attributed to the differences in climatic conditions. While mean temperature, wind speed and daily sunshine in 25 growing period was lower than in 26, precipitation and mean relative humidity in 25 growing period was higher than in 26 (Table 1). In the control treatment, T-1, the amount of total irrigation water applied and seasonal ET a values were 349.6, mm in 25, 548.5, and mm in 26, respectively. As expected, the highest seasonal ET a occurred in the T-1 obviously owing to an adequate soil water supply during the growing period. Other treatments underwent water deficits and pro-

7 488 Y. Kuslu, U. Sahin, T. Tunc and F. M. Kiziloglu Table 3 Total number of irrigation, amount of irrigation water, seasonal actual evapotranspiration (ETa), total dry forage yield (DFY) and water use efficiency (WUE) values of alfalfa in the growing periods of 25 and 26 Year Treatment Number of irrigation Irrigation water applied, mm Water saving, % Seasonal Eta, mm Total DFY, ton ha -1 WUE, kg ha -1 mm T b* 13.9 T c 12.9 T e 1.8 T f 1.3 T g 9.3 T h T a 14.9 T b 13.2 T d 11.9 T f 1.5 T gh 9.9 T ı 9. * Different letters in the column indicate significant differences at P<.1 using Duncan s multiple range test duced lower seasonal ET a. The lowest seasonal ET a was observed in the continuous stress treatments (T- ), mm in 25 and mm in 26. The decreasing ratio of seasonal ET a by the increasing water deficit in 26 growing period was higher than in 25. This situation could be explained by higher water requirement in 26 (Table 3). The seasonal ET a of the full-irrigated alfalfa plants in this study was low compared to that reported by FAO (22a), but was similar to those obtained by Sahin and Hanay (1996) and Evren and Sevim (1998) in Erzurum conditions. As shown in Table 3, data obtained from the 2- year study showed that alfalfa total dry forage yield was significantly (P<.1) affected by water deficit, and the highest values were obtained in T-1. Increasing water deficits resulted in a relatively lower plant yields. While the water saving in our study was 2% (T-8), 4% (T-6), 6% (T-4), 8% (T-2) and 1% (T-) of T-1, the rates of decreases in average dry forage yield for 2 years were found to be 22.4, 42.6, 56.8, 67.6 and 77.4% of the T-1, respectively. Therefore, it was observed that the ratio of decreases in total dry forage yield for each percent deficit rate was not constant. Frate and Roberts (26) reported that alfalfa hay yields were greatly reduced by deficit irrigation. Guitjens (1993) indicated that generally, alfalfa yields were significantly less for non-irrigation conditions. Orloff et al. (24) reported severe yield losses when irrigation was halted in summer. Hanson et al. (27) also determined similar results. The relationships between seasonal ET a (mm) and total dry forage yield (ton ha -1 ) have been evaluated for 25, 26 and The relationship between seasonal ET a and total dry forage yield was linear (P<.1) (Figure 2). This result agree with those of Bauder et al. (1987), Sheafer et al. (1988), Guitjens (199), Smeal et al. (1991), Grimes et al. (1992), Saeed and El-Nadi (1997) and Bai and Li (23).

8 Determining Water-yield Relationship, Water Use Efficiency, Seasonal Crop and Pan Coefficients Water Use Efficiency (WUE) WUE calculated by dividing total dry forage yield (kg ha -1 ) to seasonal ET a (mm) were different depending on the treatments (Table 3). The lowest WUE values were determined for continuous stress conditions (T-) in both years. WUE was increased by increasing irrigation. This could be explained that dry matter accumulation was increased by irrigation. The WUE of the full-irrigated alfalfa plants in this study was low to those reported by Grimes et al. (1992) and Hirth et al. (21) for cooler climates, but it was high to those obtained by Saeed and El-Nadi (1997) and Hirth et al. (21) for warmer climates. Yield Response Factor (k y ) Yield response factor (k y ) was determined for 25, 26 and Total dry forage yield (DFY) and seasonal actual evapotranspiration (ET a ) presented in Table 3 were used to determine relative yield reduction (Y d ) and relative evapotranspirasyon reduction (ET d ). A linear regression equation was fitted to the data (Figure 3). The k y values of alfalfa to water deficit for the entire growing season were 1.47 and 1.24 in 25 and 26, respectively. According to the regression equation, k y was 1.33, when the experimental years were considered together. The coefficient of determination (r 2 ) was.97 and the relationship was statistically significant at the level of P<.1. The k y value of 1.33 obtained in this study was higher than k y value of 1.1 reported by Doorenbos and Kassam (1979) and FAO (22a). Crop and Pan Coefficients (k c and k p ) ET a determined by the soil water balance equation, calculated with the FAO Penman Monteith equation and E pan values measured with a Class A pan during the 25 and 26 growing periods of alfalfa were given as means of 1 days in Figure 4. ET a values varied from 1.21 to 6.21 mm day -1 in 25. It was mm day -1 in 26. values varied from 2.5 to 7.11 mm day -1 in 25 and from 2.19 to 7.51 mm day -1 in 26. Seasonal was determined as 72.2 mm in 25 and mm in 26. Seasonal was higher than seasonal ET a in both years. E pan values varied from 1.88 to 7.8 mm day -1 in 25 and from 1.76 to 1.4 mm day -1 in 26. Seasonal E pan was determined as 77.7 mm in 25 and 92.5 mm in 26. Seasonal E pan was higher than seasonal in both years. Total dry forage yield, ton ha y =.19x ; r 2 =.979** (25) y =.17x ; r 2 =.982** (26) y =.18x ; r 2 =.974** (25-26) Seasonal ET a (mm) Fig. 2. Total dry forage yield of alfalfa as a function of seasonal actual evapotranspiration (ET a ) Y d = 1.47ET d ; r 2 =.973** (25) Y d = 1.24ET d ; r 2 =.97** (26) Y d = 1.33ET d ; r 2 =.973** (25-26) Yd = 1 - (Ya/Ym) ET d = 1 - (ET a /ET m ) Fig. 3. Yield response factor, k y

9 49 Y. Kuslu, U. Sahin, T. Tunc and F. M. Kiziloglu ETa, ETo, Epan, mm day ETa 26-ETa 25-ETo 26-ETo 25-Epan 26-Epan Date, days after sowing Fig. 4. Actual evapotranspiration (ET a ), reference evapotranspiration ( ) and Class A pan evaporation (E pan ) values during the 25 and 26 growing periods ETo, mm day =.9E pan ; r 2 =.754** (25) =.83E pan ; r 2 =.854** (26) =.86E pan ; r 2 =.791** (25-26) Epan, mm day -1 Fig. 6. The relationship between seasonal and E pan The values of k c derived from ET a and ET data in o Figure 4 are shown in Figure 5. The seasonal k c value was determined as.83 in 25, as.88 in 26 and as.86 in The coefficient of determination (r 2 ) was.66 in 25,.63 in 26 and.66 in 25-26; and the relationships were statistically significant at the level of P<.1. The k c value of.86 obtained in this study is a result of the ET a lower than values (Figure 4). The k c value determined from this study was similar to those suggested as by Doorenbos and Kassam (1979), Brouwer and Eta, mm day ET a =.83 ; r 2 =.663** (25) ET a =.88 ; r 2 =.627** (26) ET a =.86 ; r 2 =.662** (25-26) ETo, mm day -1 Fig. 5. The relationship between seasonal ET a and Et o Heibloem (1986), Allen et al. (1998) and FAO (22a). The values of k p was computed from and E pan data in Figure 4, and was shown in Figure 6. The seasonal k p values was determined as.9 in 25, as.83 in 26 and as.86 in The coefficient of determination (r 2 ) was.75 in 25,.85 in 26 and.79 in 25-26; and the relationships were statistically significant at the level of P<.1. The k p value of.86 obtained in this study is a result of the lower than E pan values (Figure 4). Based on earlier investigations, the FAO sources state that the k p values vary between.35 and.85 (Allen et al., 1998). Conclusion According to the results of a 2-year study, alfalfa dry forage yield was significantly decreased by water stress. Water use efficiency values were decreased by the increase in water deficit. Positive linear relationship was obtained between evapotranspiration and dry forage yield. The yield response factor (k y ) was This value could be used as a good basis for irrigation strategy development in semiarid regions with high altitude where irrigation water supplies are limited. The seasonal crop coefficient was determined as.86 for alfalfa. The value of the seasonal pan coeffi-

10 Determining Water-yield Relationship, Water Use Efficiency, Seasonal Crop and Pan Coefficients cient was.86. According to these results, the actual evapotranspiration of alfalfa under semiarid conditions with high altitude could be considered as 74 % Class A pan evaporation. References Allen, R. G., L. S. Pereira, D. Raes and M. Smith, Crop Evapotranspiration. Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper, No. 56, Rome. Bai, W. M. and L. H. Li, 23. Effect of irrigation methods and quota on root water uptake and biomass of alfalfa in the Wulanbuhe sandy region of China. Agricultural Water Management, 62: Bauder, J. W., A. Bauder, J. M. Ramirez and D. K. Cassel, Alfalfa water use and production on dryland and irrigated sandy loam. Agronomy Journal, 7: Bekele, S. and K. Tilahun, 27. Regulated deficit irrigation scheduling of onion in a semiarid region of Ethiopia. Agricultural Water Management, 89: Brouwer, C. And M. Heibloem, Irrigation water management: irrigation water needs. FAO Training Manual, No.3, Rome. Clarke, D., M. Smith and K. El-Askari, CropWat for Windows (Version 4.2). Southampton University, UK. Cuiping, X., L. Honglu and Z. Lixin, 25. Calculating alfalfa irrigation quota by FAO Penman- Monteith equation. Trans. Chinese Society of Agricultural Engineering, 21: Doorenbos, J. and A. H. Kassam, Yield Response to Water. FAO 33, Rome. English, M. and S. J. Raja, Perspectives on deficit irrigation. Agricultural Water Management, 32: Evren, S. and Z. Sevim, Water consumption of alfalfa grown in Erzurum plain. General Directorate of Rural Services. Soil and Water Resources Research Department, Publication No. 16, Ankara; FAO, 22a. Crop Water Management-Alfalfa. Land and Water Development Division, FAO, Rome. FAO, 22b. Deficit irrigation practices. FAO Water Report No. 22, Rome. Frate, C. and B. Roberts, 26. Managing alfalfa production with limited irrigation water. In: Proceedings of the 26 Western Alfalfa & Forage Conference, December, Reno, Nevada. Grimes, D. W., P. L. Wiley and W. R. Sheesley, Alfalfa yield and plant water relations with variable irrigation. Crop Science, 32: Grismer, M. E., M. Orang, R. Snyder and R. Matyac, 22. Pan evaporation to reference evapotranspiration conversion methods. Journal of Irrigation and Drainage Engineering, 128: Guitjens, J. C., 199. Alfalfa. In Stewart BA, Nielsen DR (eds.) Irrigation of Agricultural Crops. Agronomy No.3, ASA, CSSA, and SSSA, Madison, Wisconsin; pp Guitjens, J. C., Alfalfa irrigation during drought. Journal of Irrigation and Drainage Engineering, 119: Hanson, B., D. Putnam and R. Snyder, 27. Deficit irrigation of alfalfa as a strategy for providing water for water-short areas. Agricultural Water Management, 93: Hirth, J. R., P. J. Haines, A. M. Ridley and K. F. Wilson, 21. Lucerne in crop rotations on the Riverine Plains 2. Biomass and grain yields, water use efficiency, soil nitrogen, and profitability. Australian Journal of Agricultural Research, 52: Howell, T. A., R. H. Cuenca and K. H. Solomon, Crop Yield Response. In: Hoffman GJ, Howell TA, Solomon KH (eds.) Management of Farm Irrigation Systems. ASAE Monograph No.9, 295 Niles Road, St. Joseph, MI ; pp Irmak, S., D. Z. Haman and J. W. Jones, 22. Evaluation of Class A pan coefficients for estimating reference evapotranspiration in humid location. Journal of Irrigation and Drainage Engineering, 128: Johnson, B. L. and T. L. Henderson, 22. Water

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