Browsing by Author "Irmak, Suat"

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    Comparison of sum-of-hourly and daily time step standardized ASCE Penman-Monteith reference evapotranspiration
    (Theoretical and Applied Climatology, 2018) Djaman, Koffi; Irmak, Suat; Sall, Mamadou; Sow, Abdoulaye; Kabenge, Isa
    The objective of this study was to quantify differences associated with using 24-h time step reference evapotranspiration (ETo), as compared with the sum of hourly ETo computations with the standardized ASCE Penman-Monteith (ASCE-PM) model for semi-arid dry conditions at Fanaye and Ndiaye (Senegal) and semiarid humid conditions at Sapu (The Gambia) and Kankan (Guinea). The results showed that there was good agreement between the sumof hourly ETo and daily time step ETo at all four locations. The daily time step overestimated the daily ETo relative to the sum of hourly ETo by 1.3 to 8% for the whole study periods. However, there is location and monthly dependence of the magnitude of ETo values and the ratio of the ETo values estimated by both methods. Sum of hourly ETo tends to give higher ETo during winter time at Fanaye and Sapu, while the daily ETo was higher from March to November at the same weather stations. At Ndiaye and Kankan, daily time step estimates of ETo were high during the year. The simple linear regression slopes between the sum of 24-h ETo and the daily time step ETo at all weather stations varied from 1.02 to 1.08 with high coefficient of determination (R2 ≥ 0.87). Application of the hourly ETo estimation method might help on accurate ETo estimation to meet irrigation requirement under precision agriculture
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    Evaporative losses from a common reed-dominated peachleaf willow and cottonwood riparian plant community
    (Water Resources Research, 2012) Kabenge, Isa; Irmak, Suat
    Our study is one of the first to integrate and apply within-canopy radiation physics parameters and scaling-up leaf-level stomatal resistace (rL) to canopy resistance (rc) approach to quantify hourly transpiration (TRP) rates of individual riparian plant species—common reed (Phragmites australis), peachleaf willow (Salix amygdaloides), and cottonwood (Populus deltoides)— in a mixed riparian plant community in the Platte River Basin in central Nebraska. Two experimental years (2009 and 2010) were contrasted by warmer air temperature and presence of flood water in 2010. The seasonal average rc values for common reed, peachleaf willow, and cottonwood in 2009 were 76, 70, and 107 s m 1, respectively. The corresponding rc values in the flood year (2010) were 70, 66, and 105 s m 1 for the same species, respectively. In 2009, the seasonal total TRP for common reed, peachleaf willow, and cottonwood were 483, 522, and 431 mm, respectively. Corresponding TRP values in 2010 were greater as 550, 655, and 496 mm, respectively. In 2009, TRP accounted for 64% of ETa during June–September, and the proportion varied between 41% and 69% for most of the season. In 2010, TRP accounted for 61% of ETa during June–September, and the proportion varied between 41% and 65% for most of the season. The average surface evaporation rate of the riparian zone was 0.81 mm d 1 in 2009 and 1.70 mm d 1 in 2010. Seasonal evaporation was 160 mm in 2009 and 312 mm in 2010. The study provides a basis for understanding the dynamics of transpiration for riparian vegetation in response to the environmental conditions and provides valuable water use data for more complete water balance analyses by accounting for the water use of riparian vegetation species.
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    Trend and magnitude of changes in climate variables and reference evapotranspiration over 116-yr period in the Platte River Basin, central Nebraska–USA
    (Journal of Hydrology, 2012) Irmak, Suat; Kabenge, Isa; Skaggs, Kari E.; Mutiibwa, Denis
    Some studies that investigate the climate change and hydrologic balance relationships utilize reference (potential) evapotranspiration (ETref) 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 ETref 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, ETref 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 ETref rates have been changing and are expected to change in the future, single-variable equations for estimating the trend in ETref should be avoided due to the inherent nature of the trend passed to ETref 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 (Rs), net radiation (Rn), wind speed at 2-m (u2), relative humidity (RH), and vapor pressure deficit (VPD)] only from observed maximum and minimum air temperatures (Tmax and Tmin) and precipitation (P) data to be used in Penman–Monteith-type combination-based energy balance equations to predict grass-and alfalfa-reference evapotranspiration (ETo and ETr, 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 Tmin and Tavg at a rate 0.038 C yr 1 and 0.0187 C yr 1, respectively, and insignificant increase in u2 and VPD, we observed a significant (P < 0.05) decline in ETref ( 0.3596 mm yr 1 for ETo and 0.3586 mm yr 1 for ETr). We present data, analyses, and interpretation that the decrease in ETref is most likely due to significant (P < 0.05) increase in precipitation (0.87 mm yr 1) that results in significant reduction in Rs ( 0.0223 MJ m 2 yr 1) and, in turn in Rn ( 0.0032 MJ m 2 yr 1), which resulted in reduction in ETref because increase in P decreases available energy, which is primary driver of ETref. There was approximately 100 mm of increase in precipitation from 1893 to 2008 in the study location at a rate of about 0.90 mm yr 1. Also, there was a significant increase in maximum daily precipitation, especially in the very high events (i.e., >70 mm d 1). We present detailed analyses of relationships between ETref and all meteorological variables. On an annual time step ETref significantly (P < 0.05) and inversely correlated to precipitation and RH, and significantly and positively correlated to Tmax, Tavg, VPD, Rs, and Rn. We observed a higher degree of responsiveness of ETo to changes in meteorological variables than ETr, which may indicate that ETo may be more apposite to better detect the impact of changes in meteorological variables on ETref in climate change studies.