TY - JOUR
T1 - Trend and magnitude of changes in climate variables and reference evapotranspiration over 116-yr period in the Platte River Basin, central Nebraska-USA
AU - Irmak, Suat
AU - Kabenge, Isa
AU - Skaggs, Kari E.
AU - Mutiibwa, Denis
PY - 2012/2/14
Y1 - 2012/2/14
N2 - Some studies that investigate the climate change and hydrologic balance relationships utilize reference (potential) evapotranspiration (ET ref) to either calculate the changes in trends and magnitude of actual ET or to determine changes in atmospheric demand. In such cases, it is important to acquire robust ET ref estimates to correctly assess the impact of changes in meteorological variables on atmospheric evaporative demand, hydrologic balances, response of vegetation to climate, and their interactions. Despite its crucial importance, unfortunately, ET ref is sometimes poorly addressed in climate change studies as some studies utilize temperature or radiation-based empirical equations due to various reasons (unavailability of climate data to solve combination-based energy balance equations, etc.). Since many climate variables that affect ET ref rates have been changing and are expected to change in the future, single-variable equations for estimating the trend in ET ref should be avoided due to the inherent nature of the trend passed to ET ref from the variable. Here, we showed an integrated approach of practical and robust procedures that are already exist to estimate necessary climate variables [incoming shortwave radiation (R s), net radiation (R n), wind speed at 2-m (u 2), relative humidity (RH), and vapor pressure deficit (VPD)] only from observed maximum and minimum air temperatures (T max and T min) and precipitation (P) data to be used in Penman-Monteith-type combination-based energy balance equations to predict grass-and alfalfa-reference evapotranspiration (ET o and ET r, respectively). We analyzed the trends and magnitudes of change in meteorological variables for a 116-yr period from 1893 to 2008 in the agro-ecosystem-dominated Platte River Basin in central Nebraska, USA. Although we found a significant (P<0.05) increase in T min and T avg at a rate 0.038°Cyr -1 and 0.0187°Cyr -1, respectively, and insignificant increase in u 2 and VPD, we observed a significant (P<0.05) decline in ET ref (-0.3596mmyr -1 for ET o and -0.3586mmyr -1 for ET r). We present data, analyses, and interpretation that the decrease in ET ref is most likely due to significant (P<0.05) increase in precipitation (0.87mmyr -1) that results in significant reduction in R s (-0.0223MJm -2yr -1) and, in turn in R n (-0.0032MJm -2yr -1), which resulted in reduction in ET ref because increase in P decreases available energy, which is primary driver of ET ref. There was approximately 100mm of increase in precipitation from 1893 to 2008 in the study location at a rate of about 0.90mmyr -1. Also, there was a significant increase in maximum daily precipitation, especially in the very high events (i.e., >70mmd -1). We present detailed analyses of relationships between ET ref and all meteorological variables. On an annual time step ET ref significantly (P<0.05) and inversely correlated to precipitation and RH, and significantly and positively correlated to T max, T avg, VPD, R s, and R n. We observed a higher degree of responsiveness of ET o to changes in meteorological variables than ET r, which may indicate that ET o may be more apposite to better detect the impact of changes in meteorological variables on ET ref in climate change studies.
AB - Some studies that investigate the climate change and hydrologic balance relationships utilize reference (potential) evapotranspiration (ET ref) to either calculate the changes in trends and magnitude of actual ET or to determine changes in atmospheric demand. In such cases, it is important to acquire robust ET ref estimates to correctly assess the impact of changes in meteorological variables on atmospheric evaporative demand, hydrologic balances, response of vegetation to climate, and their interactions. Despite its crucial importance, unfortunately, ET ref is sometimes poorly addressed in climate change studies as some studies utilize temperature or radiation-based empirical equations due to various reasons (unavailability of climate data to solve combination-based energy balance equations, etc.). Since many climate variables that affect ET ref rates have been changing and are expected to change in the future, single-variable equations for estimating the trend in ET ref should be avoided due to the inherent nature of the trend passed to ET ref from the variable. Here, we showed an integrated approach of practical and robust procedures that are already exist to estimate necessary climate variables [incoming shortwave radiation (R s), net radiation (R n), wind speed at 2-m (u 2), relative humidity (RH), and vapor pressure deficit (VPD)] only from observed maximum and minimum air temperatures (T max and T min) and precipitation (P) data to be used in Penman-Monteith-type combination-based energy balance equations to predict grass-and alfalfa-reference evapotranspiration (ET o and ET r, respectively). We analyzed the trends and magnitudes of change in meteorological variables for a 116-yr period from 1893 to 2008 in the agro-ecosystem-dominated Platte River Basin in central Nebraska, USA. Although we found a significant (P<0.05) increase in T min and T avg at a rate 0.038°Cyr -1 and 0.0187°Cyr -1, respectively, and insignificant increase in u 2 and VPD, we observed a significant (P<0.05) decline in ET ref (-0.3596mmyr -1 for ET o and -0.3586mmyr -1 for ET r). We present data, analyses, and interpretation that the decrease in ET ref is most likely due to significant (P<0.05) increase in precipitation (0.87mmyr -1) that results in significant reduction in R s (-0.0223MJm -2yr -1) and, in turn in R n (-0.0032MJm -2yr -1), which resulted in reduction in ET ref because increase in P decreases available energy, which is primary driver of ET ref. There was approximately 100mm of increase in precipitation from 1893 to 2008 in the study location at a rate of about 0.90mmyr -1. Also, there was a significant increase in maximum daily precipitation, especially in the very high events (i.e., >70mmd -1). We present detailed analyses of relationships between ET ref and all meteorological variables. On an annual time step ET ref significantly (P<0.05) and inversely correlated to precipitation and RH, and significantly and positively correlated to T max, T avg, VPD, R s, and R n. We observed a higher degree of responsiveness of ET o to changes in meteorological variables than ET r, which may indicate that ET o may be more apposite to better detect the impact of changes in meteorological variables on ET ref in climate change studies.
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U2 - 10.1016/j.jhydrol.2011.12.006
DO - 10.1016/j.jhydrol.2011.12.006
M3 - Article
AN - SCOPUS:84856223834
SN - 0022-1694
VL - 420-421
SP - 228
EP - 244
JO - Journal of Hydrology
JF - Journal of Hydrology
ER -
