## Variation in reference crop evapotranspiration caused by the Ångström-Prescott coefficient: locally calibrated versus the FAO recommended

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**Variation in reference crop evapotranspiration caused by the Ångström-Prescott coefficient : locally calibrated versus the FAO recommended.** / Liu, Xiaoying; Mei, Xurong; Li, Yuzhong; Wang, Qingsuo; Zhang, Yanging; Porter, John Roy.

Publikation: Bidrag til tidsskrift › Tidsskriftartikel › Forskning › fagfællebedømt

#### Harvard

*Agricultural Water Management*, bind 96, nr. 7, s. 1137-1145. https://doi.org/10.1016/j.agwat.2009.03.005

#### APA

*Agricultural Water Management*,

*96*(7), 1137-1145. https://doi.org/10.1016/j.agwat.2009.03.005

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#### RIS

TY - JOUR

T1 - Variation in reference crop evapotranspiration caused by the Ångström-Prescott coefficient

T2 - locally calibrated versus the FAO recommended

AU - Liu, Xiaoying

AU - Mei, Xurong

AU - Li, Yuzhong

AU - Wang, Qingsuo

AU - Zhang, Yanging

AU - Porter, John Roy

PY - 2009

Y1 - 2009

N2 - Accurate estimation of the reference crop evapotranspiration (ET0) is investigated due to its critical role in affecting calculation of crop water use and efficiency in agricultural ecosystems. The main emphasis in this paper is to clarify the possible uncertainty in the estimation of ET0 associated with using un-calibrated Ångström-Prescott (A-P) coefficients. We first calibrated the coefficients using long-term data records from 34 sites in the Yellow River basin in China, and then applied these coefficients to estimate short wave irradiance (Rs) and ET0 at 16 sites to evaluate the difference in ET0 between the FAO recommended and the locally calibrated. We found that the direct use of the FAO recommended coefficients significantly affected the estimation of ET0 at most sites, which differed from -3% to 15% at daily scale and from -4% to 16% at monthly scale from the locally calibrated ones. These differences are comparable with or larger than those caused by some alternatives of the FAO recommended algorithms for net irradiance or vapor pressure, which further highlights the importance of using the locally calibrated coefficients. The degree of difference in ET0 showed a significant threshold relation with altitude and longitude in such a way that relatively small impact lies around 2233 m and 98°E, and away from these, the effect begins to increase. Given the large overestimation in water use as a consequence of the significant overestimation in ET0 associated with the direct use of the FAO coefficients, especially in those high yield production areas with altitude <1200 m, we developed several relationships between the A-P coefficient a, b, (a + b) and other easily obtainable factors (altitude, longitude and air temperature). A three-step procedure was recommended in applying these relations, which was (1) determine if calibration is needed or not for a given location; (2) estimate one of the A-P coefficients, either a or b if calibration is needed; (3) estimate the remaining coefficient using relations of (a + b) due to its higher coefficient of determination. In summary, we have revealed the errors and areas that are most affected when using the un-calibrated coefficients, and discussed the consequence of such error on agricultural production, and proposed practical solutions to avoid large errors. These results are intended to make the research community aware of such errors so that more appropriate choice of these coefficients is made. We hope that similar assessment will be done in other climates, contributing to managing water resources efficiently in water basins.

AB - Accurate estimation of the reference crop evapotranspiration (ET0) is investigated due to its critical role in affecting calculation of crop water use and efficiency in agricultural ecosystems. The main emphasis in this paper is to clarify the possible uncertainty in the estimation of ET0 associated with using un-calibrated Ångström-Prescott (A-P) coefficients. We first calibrated the coefficients using long-term data records from 34 sites in the Yellow River basin in China, and then applied these coefficients to estimate short wave irradiance (Rs) and ET0 at 16 sites to evaluate the difference in ET0 between the FAO recommended and the locally calibrated. We found that the direct use of the FAO recommended coefficients significantly affected the estimation of ET0 at most sites, which differed from -3% to 15% at daily scale and from -4% to 16% at monthly scale from the locally calibrated ones. These differences are comparable with or larger than those caused by some alternatives of the FAO recommended algorithms for net irradiance or vapor pressure, which further highlights the importance of using the locally calibrated coefficients. The degree of difference in ET0 showed a significant threshold relation with altitude and longitude in such a way that relatively small impact lies around 2233 m and 98°E, and away from these, the effect begins to increase. Given the large overestimation in water use as a consequence of the significant overestimation in ET0 associated with the direct use of the FAO coefficients, especially in those high yield production areas with altitude <1200 m, we developed several relationships between the A-P coefficient a, b, (a + b) and other easily obtainable factors (altitude, longitude and air temperature). A three-step procedure was recommended in applying these relations, which was (1) determine if calibration is needed or not for a given location; (2) estimate one of the A-P coefficients, either a or b if calibration is needed; (3) estimate the remaining coefficient using relations of (a + b) due to its higher coefficient of determination. In summary, we have revealed the errors and areas that are most affected when using the un-calibrated coefficients, and discussed the consequence of such error on agricultural production, and proposed practical solutions to avoid large errors. These results are intended to make the research community aware of such errors so that more appropriate choice of these coefficients is made. We hope that similar assessment will be done in other climates, contributing to managing water resources efficiently in water basins.

KW - BRIC

KW - Reference crop evapotranspiration

KW - Ångström_prescott coeffients

KW - Penman-Monteith

U2 - 10.1016/j.agwat.2009.03.005

DO - 10.1016/j.agwat.2009.03.005

M3 - Journal article

VL - 96

SP - 1137

EP - 1145

JO - Agricultural Water Management

JF - Agricultural Water Management

SN - 0378-3774

IS - 7

ER -

ID: 12487997