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Civil Engineering

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Journal of Irrigation and Drainage Engineering

Closure to “Evaluation of the Vapor Pressure Models in the Estimation of Actual Vapor Pressure and Evapotranspiration” by Hitesh Upreti and C. S. P. Ojha

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Journal of Hazardous, Toxic and Radioactive Waste

Climate Change and Sustainable Water Resources Management

In this study, remote sensing-based algorithm, METRIC is used to estimate evapotranspiration (ET) over a mountainous watershed in the lesser Himalayas in Uttarakhand, India. ET estimates of METRIC are compared with the ET values determined by using FAO Penman-Monteith equation and crop coefficients. In its original form, METRIC underestimated the ET values considerably with mean absolute error (MAE) of 0.95 mm/day. A modification is incorporated in METRIC for better representation of the effect of rainfall on the ET process. The proposed modification estimates ET at the hot pixel using daily rainfall and meteorological data. Using the modification, the MAE is reduced to 0.44 mm/day and underestimation of ET is reduced from 20.9% to 8.4%. This study contributes to the better understanding of ET in the lesser Himalayas of Uttarakhand. It also underlines the importance of accurate assignment of ET to the hot pixel for accurate estimation of ET using METRIC.

Geocarto International

Estimation of evapotranspiration in lesser Himalayas using remote sensing based surface energy balance algorithm

{\textcopyright} 2018 American Society of Civil Engineers. The quantification of evapotranspiration (ET) is highly important in water resources planning. The most reliable method for determining ET is to measure it in the field using lysimeters. However, the use of lysimeters for measurement of crop ET (ETc) is both expensive and time consuming, thus necessitating a cheaper yet reliable alternative method. One such method is to determine ET using the Penman-Monteith equation, which requires daily meteorological data as inputs. Three vapor pressure models (Models 1-3) are mentioned in Irrigation and Drainage Paper-56 of the Food and Agriculture Organization (FAO-56) for the estimation of actual vapor pressure (ea) that is used in the estimation of reference ET (ET0) by the FAO Penman-Monteith equation. Model 1, which uses daily maximum and minimum temperature and relative humidity (RH) data, is the preferred method to estimate ea; other models are used in data-constrained situations. Another vapor pressure model (Model 5) is proposed in this study. The performance of the vapor pressure models along with two other models (Models 4 and 6) existing in the literature is compared with that of Model 1 in estimating daily ea and ET0 values. The ea values were underestimated by Model 2, whereas Models 3, 4 and 5 overestimated ea. Results show that Models 4 and 5 (proposed model) performed better than FAO-56 Models 2 and 3 for estimating daily ea values for Roorkee station, India. However, Model 4, which uses only daily mean temperature and RH data, has lower accuracy in ET0 estimation when compared to Model 5. This is because Model 4 underestimated the daily saturation vapor pressure values. This suggests that daily mean temperature is not enough for the accurate estimation of daily ET0 values. The overall performance of Model 5, which uses the Lawrence dewpoint temperature-RH relationship, was more consistent and closest to Model 1 when compared to the other vapor pressure models for the study region in data-constrained situations. Furthermore, the effect of wind speed ranges on the performance of the vapor pressure models in ET0 estimation is also assessed. The effect of vapor pressure models in ET0 estimation is not considerable for light winds but becomes substantial for higher wind speeds.

Evaluation of the Vapor Pressure Models in the Estimation of Actual Vapor Pressure and Evapotranspiration

AGU Fall Meeting Abstracts

Numerical Model for Soil Moisture Simulation in Cropped Area

Sustainable Water Resources Management

Optimal Use of Irrigation Water

{\textcopyright} 2017 American Society of Civil Engineers. Hydrological processes such as evaporation and evapotranspiration (ET) are affected by air humidity. Air humidity is commonly expressed as relative humidity (RH) or dew point temperature (Tdew) and is required for the determination of evaporation from water or soil surfaces and ET from cropped areas. However, the availability of humidity data is sometimes an issue. In cases in which RH data are not available, air humidity on a daily basis can be estimated by using daily maximum and minimum temperatures along with the dew point temperature. Guidelines from an international organization recommend the use of a dew point temperature slightly less than the daily minimum temperature for daily computations, when its measurements are not available. In this study, the daily average dew point temperature (Tdewmean) was taken as k°C less than the daily minimum temperature. Thus, daily RH data (maximum, minimum and mean RH) were generated from daily maximum and minimum temperatures for Roorkee Station (India) during 2012-2016. Four different values of k (k = 0, 1, 2, 3) were used for determining the actual vapor pressure values used in RH estimation. Daily RH values determined using different values of k were compared with observed RH data. It was observed that for the nonsummer months, having an average value of daily mean RH greater than 60% (January-March and July-December), k = 0 resulted in minimum error in RH estimation. However, during the summer months, having an average value of daily mean RH less than 60% (April-June), k = 3 provided relatively better estimates of RH. During the summer months, large errors occurred in the estimation of maximum and minimum RH values using this approach; hence, mean RH values that are relatively better estimated during these periods should be used for hydrological applications. Further, linear Tdew-RH relationships (with and without parabolic correction) have been assessed to determine daily Tdewmeanvalues. These relationships have been found to provide more accurate estimates of Tdewmean and actual vapor pressure on a daily basis than the FAO-56 vapor pressure models when only daily RHmean data are available (instead of RHmax and RHmindata) along with daily temperature data.

Estimation of Relative Humidity and Dew Point Temperature Using Limited Meteorological Data

The most reliable method used to estimate deep percolation is using Lysimeter which proves to be expensive and time consuming as well. In this study, deep percolation has been estimated by using water balance method. The experimental setup consisted of two lysimeters in the Hydraulic Engineering field lab, I.I.T., Roorkee in which maize crop was grown. Regular measurements of soil moisture were made at the depths 0-20 cm, 20-40 cm, 40- 60 cm, 60-80 cm and 80-100 cm using gravimetric method. The evapotranspiration estimates were determined using Penman-Moneith equation. The deep percolation is calculated using the water-balance approach. The deep percolation calculated using the water-balance approach is comparable to the observed values of deep percolation which are obtained from the lysimeters.