Preliminary Performance Evaluation of the Penman Monteith Evapotranspiration Equation in Southeastern Colorado (original) (raw)
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Agronomy Journal, 2019
Potential evapotranspiration (PET) is a key variable simulated by most crop simulation models using a variety of approaches. The objective of this study was to compare Priestley-Taylor (PT) and FAO-56 Penman-Monteith (FAO-56 PM) PET methods when simulating crop evapotranspiration (ET), yield, and aboveground biomass in Oklahoma. The study used data from 87 weather stations across nine climate divisions to simulate maize, sorghum, soybean, and wheat crop growth and development in Oklahoma for 1998 to 2017. Our results show that seasonal crop ET estimated by PT was lower than FAO-56 PM in most climate divisions and crops with average difference ranging from-10 to-1% for rainfed and from-21 to-1% for irrigated simulations. Differences in ET were greater for winter wheat than for maize, sorghum, and soybean. Additionally, differences in ET between methods were smaller in humid regions than in arid regions. Analysis of simulated rainfed yield and biomass showed higher values with PT for all crops except in the most humid climate divisions. However, under full irrigation, the yield differences between PT and FAO-56 PM were very low and ranged between 0-2% for all crops. In conclusion, this study confirmed that PT estimation of ET could be significantly different from FAO-56 PM especially in the arid and semiarid regions and during the winter under rainfed conditions. However, the differences in ET estimation did not affect yield and biomass simulation under full irrigation because the impact of soil water balance on the crop growth simulation was removed.
2011
Wind speed adjustment………………………………………………… Fetch requirement……………………………………………………… Evaluation and statistical analysis……………………………………... RESULTS AND DISCUSSION………………………………..…………….. Neutron Moisture readings…………………………………………….. Crop height and area adjustment………………………………………. Hourly measured ET ref versus calculated ET rs ……….………………… Daily measured ET ref versus ET rs ……………………………………… Residual analysis……………………………………………..………… Effect of footprint length on evapotranspiration……………………….. CONCLUSION………………………………………….……….…………… REFERENCES………………………………………………………………… CHAPTER THREE: Alfalfa crop coefficients developed using a weighing lysimeter in southeast Colorado………………………………..……………… ABSTRACT………………………………………………………….………… INTRODUCTION……………………………………………………………... vii MATERIALS AND METHODS……………………………………….……… Experimental location ………………………………………….……… Lysimeter design……………………………………….……….……… Climate and soil measurements………………………………....…...… Alfalfa and irrigation management……………………………..……… Water balance ………………………………………………….……… Reference ET calculation……………………………………….……… Growing Degree Days (GDD) ………………………………………… Crop coefficient (K cr) calculations…………………………….……..… RESULTS AND DISCUSSION……………………………………….……… Seasonal Water balance………………..……………………….……… Alfalfa Evapotranspiration………..………….………………………… Alfalfa crop coefficients……….……………………………….………
Watershed Management and Operations Management 2000, 2004
In 1998, the Food and Agriculture Organization of the United Nations (FAO) published FAO Irrigation and Drainage Paper No. 56, a revision of the earlier and widely used Paper No. 24 for calculating evapotranspiration (ET) and crop water requirements. The revision uses a single method, the FAO Penman-Monteith equation, for calculating reference evapotranspiration (ET o). In addition to the "mean" crop coefficient (K c) values of FAO-24, FAO-56 provides tables of "basal" crop coefficients that represent ET under conditions having a dry soil surface. Associated equations for predicting evaporation from bare soil associated with crop transpiration are based on a water balance of the soil surface layer. Comparisons of daily ET from three agricultural crops are made between lysimeter measured ET and the basal K c method of FAO-56 and the time-based basal K c procedure of Wright (1982). Standard errors of estimate and accuracies were similar between the two methods and averaged about 0.77 mm/day or 15%.
Theoretical and Applied Climatology, 2018
Reference evapotranspiration (ETo) is an important parameter in hydrological, agricultural, and environmental studies. Accurate estimation of ETo helps to improve water management and increase water productivity and efficiency. While the Penman-Monteith ETo equation enjoys worldwide adoption as the most accurate ETo equation, the number of requested climatic variables makes its application very questionable under limited data conditions. The objective of this study was to evaluate the Penman-Monteith ETo equation under limited climatic data and 34 simple ETo equations that request few climatic variables. Five weather stations were considered under the semiarid and dry climate across New Mexico for the period of 2009-2017. The Penman-Monteith ETo equation showed good performance under missing solar radiation, relative humidity, and wind speed and could still be adapted under limited data conditions across New Mexico. However, it tended to underestimate daily ETo when more than one climatic variable data is missing. Among the simple ETo equations, four of the Valiantzas equations, along with the Makkink, Calibrated Hargreaves, Abtew, Jensen-Haise, and Caprio equations, were the best performing ones compared to the Penman-Monteith equation and could be the best alternative ETo estimation methods. These alternative equations could be used by irrigation managers, producers, engineers, and university researchers to improve water management across the dry semiarid and arid zone across New Mexico, as well as other semiarid areas where water is the most limiting factor to food and fiber production.
Applied Engineering in Agriculture, 2005
Estimated daily reference crop evapotranspiration (ET o ) is normally used to determine the water requirement of crops using the crop factor method. Many ET o estimation methods have been developed for different types of climatic data, and the accuracy of these methods varies with climatic conditions. In this study, pair−wise comparisons were made between daily ET o estimated from eight different ET o equations and ET o measured by lysimeter to provide information helpful in selecting an appropriate ET o equation for the Cumberland Plateau located in the humid Southeast United States. Based on the standard error of the estimate (S yx ), the relationship between the estimated and measured ET o was the best using the FAO−56 Penman−Monteith equation (coefficient of determination (r 2 ) = 0.91, S yx = 0.31 mm d −1 , and a coefficient of efficiency (E) = 0.87), followed by the Penman (1948) equation (r 2 = 0.91, S yx = 0.34 mm d −1 , and E = 0.88), and Turc's equation (r 2 = 0.90, S yx = 0.36 mm d −1 , and E = 0.88). The FAO−24 Penman and Priestly−Taylor methods overestimated ET o , while the Makkink equation underestimated ET o . The results for the Hargreaves−Samani equation showed low correlation with lysimeter ET o data (r 2 = 0.51, S yx = 0.68 mm d −1 , and E = 0.20), while those for the Kimberly Penman were reasonable (r 2 = 0.87, S yx = 0.40 mm d −1 , and E = 0.87). These results support the adoption of the FAO−56 Penman−Monteith equation for the climatological conditions occurring in the humid Southeast. However, Turc's equation may be an attractive alternative to the more complex Penman−Monteith method. The Turc method requires fewer input parameters, i.e., mean air temperature and solar irradiance data only.
FAO-56 Dual Crop Coefficient Method for Estimating Evaporation from Soil and Application Extensions
Journal of Irrigation and Drainage Engineering, 2005
Crop coefficient curves provide simple, reproducible means to estimate crop evapotranspiration (ET) from weather-based reference ET values. The dual crop coefficient ͑K c ͒ method of the Food and Agricultural Organization of the United States (FAO) Irrigation and Drainage Paper No. 56 (FAO-56) is intended to improve daily simulation of crop ET by considering separately the contribution of evaporation from soil. The dual method utilizes "basal" crop coefficients representing ET from crops having a dry soil surface and separately predicts evaporation from bare soil based on a water balance of the soil surface layer. Three extensions to the evaporation calculation procedure are described here that are intended to improve accuracy when applications warrant the extra complexity. The first extension uses parallel water balances representing the portion of the soil surface wetted by irrigation and precipitation together and the portion wetted by precipitation alone. The second extension uses three "stages" for surface drying and provides for application to deep cracking soils. The third extension predicts the extraction of the transpiration component from the soil surface layer. Sensitivity and analyses and illustrations indicate moderate sensitivity of daily calculated ET to application of the extensions. The dual K c procedure, although relatively simple computationally and structurally, estimates daily ET as measured by lysimeter relatively well for periods of bare soil and partial and full vegetation cover.
Journal of Irrigation and Drainage Engineering-asce, 2008
Alfalfa-reference evapotranspiration ͑ET r ͒ values sometimes need to be converted to grass-reference ET ͑ET o ͒, or vice versa, to enable crop coefficients developed for one reference surface to be used with the other. However, guidelines to make these conversions are lacking. The objectives of this study were to: ͑1͒ develop ET r to ET o ratios ͑K r values͒ for different climatic regions for the growing season and nongrowing ͑dormant͒ seasons; and ͑2͒ determine the seasonal behavior of K r values between the locations and in the same location for different seasons. Monthly average K r values from daily values were developed for Bushland, ͑Tex.͒, Clay Center, ͑Neb.͒, Davis, ͑Calif.͒, Gainesville, ͑Fla.͒, Phoenix ͑Ariz.͒, and Rockport, ͑Mo.͒ for the calendar year and for the growing season ͑May-September͒. ET r and ET o values that were used to determine K r values were calculated by several methods. Methods included the standardized American Society of Civil Engineers Penman-Monteith ͑ASCE-PM͒, Food and Agriculture Organization Paper 56 ͑FAO56͒ equation ͑68͒, 1972 Kimberly-Penman, 1963 Jensen-Haise, and the High Plains Regional Climate Center ͑HPRCC͒ Penman. The K r values determined by the same and different methods exhibited substantial variations among locations. For example, the K r values developed with the ASCE-PM method in Phoenix, and Rockport, respectively. The variability in the K r values among locations justifies the need for developing local K r values because the values did not appear to be transferable among locations. In general, variations in K r values were less for the growing season than for the calendar year. Average standard deviation between years was maximum 0.13 for the calendar year and maximum 0.10 for the growing season. The ASCE-PM K r values had less variability among locations than those obtained with other methods. The FAO56 procedure K r values had higher variability among locations, especially for areas with low relative humidity and high wind speed. The 1972 Kim-Pen method resulted in the closest K r values compared with the ASCE-PM method at all locations. Some of the methods, including the ASCE-PM, produced potentially unrealistically high K r values ͑e.g., 1.78, 1.80͒ during the nongrowing season, which could be due to instabilities and uncertainties that exist when estimating ET r and ET o in dormant season since the hypothetical reference conditions are usually not met during this period in most locations. Because simultaneous and direct measurements of the ET r and ET o values rarely exist, it appears that the approach of ET r to ET o ratios calculated with the ASCE-PM method is currently the best approach available to derive K r values for locations where these measurements are not available. The K r values developed in this study can be useful for making conversions from ET r to ET o , or vice versa, to enable using crop coefficients developed for one reference surface with the other to determine actual crop water use for locations, with similar climatic characteristics of this study, when locally measured K r values are not available.
2020
Crop water requirement, a key component for Irrigation planning and management depends on Actual Evapotranspiration rates. Variations in Evapotranspiration rates depends on the climatic conditions for a given soil and crop. The objective of this work is to determine the water consumptive use based on crop coefficients for Tomato in Semiarid and Sub-humid agro climates. The Actual Evapotranspiration was quantified by Lysimeters. Sieve analysis of the soil indicated as sandy soil and has density of 1.859*10-3 Kg/cm 3. Depending on density and the root depth of tomato crop, lysimeter of dimensions 52cm depth and 36cm diameter is used to measure actual evapotranspiration rate. Regression analysis carried out for the actual evapotranspiraton rates, computed using empirical formula indicated that the FAO-56 PM method is well suited for both the regions having correlation coefficients of 0.94 and 0.92 for Semiarid and subhumid regions respectively. Further, it was found that Thornthwaite equation being the next suited method has a correlation of 0.90 for semiarid and Hargreaves method the next suited method with correlation of about 0.90 for sub-humid. Crop coefficients used in all this potential Evapotranspiration methods were calibrated with lysimeter insitu measurements. The crop coefficients vary depending on the different crop stages. The recalibrated crop coefficients for tomato crop are 0.
Comparison of various methods for estimating reference crop evapotranspiration
The reference crop evapotranspiration (ETo) was estimated for three locations of Bangladesh using the standard Penman-Monteith (P-M) method as recommended by the Food and Agriculture Organization of the United Nations (FAO). The same was also estimated using four other emperical methods and compared with the standard method. The regression equations developed were evaluated with independent data sets. The superiority order were found as: FAO temperature, Radiation, Hargreaves and pan evaporation, respectively. lt is revealed from the study that the regression equation developed herein with FAO temperature method can be used to estimate ETo more accurately than the original FAO temperature method, as well as regression equations with other methods. lntroduction Management practices for optimal utilization of water have been increasingly emphasized because of unevenly distributed rainfall, high evapotranspiration and excessive depletion of groundwater resources. Thus practical methods for the accurate estimation of water use in irrigated agriculture are essential. The estimation of crop water requirement is one of the principal steps in the planning, design and operation of irrigation and water resources systems. Crop water requirements vary with crop characteristics and local condition. Relationships between the evapotranspiration of a pre-selected crop (the reference crop), which is referred to as reference evapotranspiration (ETo), and other crops are established by multiplying ETo by crop coefficients. The ETs depends on local meteorological conditions, whereas the actual evapotranspiration (ET) of a crop depends on its characteristics, time of planting or sowing and stage of crop development.