Daily Kc Research Articles

Evaluation of actual evapotranspiration measured by large‐scale weighing lysimeters in a humid alpine meadow, northeastern Qinghai‐Tibetan Plateau

AbstractThe accurate estimation of evapotranspiration (ET) is essential for assessing water availability and requirements of regional‐scale terrestrial ecosystems, and for understanding the hydrological cycle in alpine ecosystems. In this study, two large‐scale weighing lysimeters were employed to estimate the magnitude and dynamics of actual evapotranspiration in a humid alpine Kobresia meadow from January 2018 to December 2019 on the northeastern Qinghai‐Tibetan Plateau (QTP). The results showed that daily ETa averaged 2.24 ± 0.10 mm day −1 throughout the study period, with values of 3.89 ± 0.14 and 0.81 ± 0.06 mm day−1 during the growing season and non‐growing season, respectively. The cumulative ETa during the study period was 937.39 mm, exceeding precipitation (684.20 mm) received at the site during the same period by 37%, suggesting that almost all precipitation in the lysimeters was returned to the atmosphere by evapotranspiration. Furthermore, the cumulative ETa (805.04 mm) was almost equal to the maximum potential evapotranspiration estimated by the FAO‐56 reference evapotranspiration (ET0) (801.94 mm) during the growing season, but the cumulative ETa (132.25 mm) was 113.72% less than the minimum equilibrium ETeq) (282.86 mm) during the non‐growing season due to the limited surface moisture in frozen soil. The crop coefficient (Kc) also showed a distinct seasonal pattern, with a monthly average of 1.01 during the growing season. Structural equation model (SEM) and boosted regression tree (BRT) show that net radiation and air temperature were the most important factors affecting daily ETa during the whole study period and growing season, but that non‐growing season ETa was dominated by soil water content and net radiation. The daily Kc was dominated by net radiation. Furthermore, both ETa and Kc were also affected by aboveground biomass.

Preliminary crop coefficients for late planted short‐season soybean: Texas High Plains

AbstractHaving similar profit potential but roughly half of the water requirements of corn (Zea mays L.), cotton (Gossypium hirsutum L.) has become an attractive crop in the Southern High Plains (SHP) where groundwater levels and well capacities continue to decline. However, unpredictable and erratic precipitation often results in conditions either too wet or too dry for ideal germination. Coupled with a relatively short planting window for adequate heat unit accumulation and the threat of severe weather events, cotton crops can fail, leaving limited options for producers wanting to plant a second summer crop. One option is short‐season soybean [Glycine max (L.) Merr.]. However, limited data on crop water use (evapotranspiration, ET), crop coefficient (Kc), and yield exist for late‐planted soybean in the semiarid SHP. After unseasonably wet conditions in May 2019, a short‐season soybean variety was planted on 13 June in irrigated weighing lysimeter fields at the USDA‐ARS Conservation and Production Research Laboratory at Bushland, TX. Results showed profitable yields (4,786–4,996 kg ha–1), and although maximum daily Kc values were not different than published values, season length was 24–29 d shorter than for soybean planted in mid‐May. Seasonal water use was less than that of archived data using earlier planting dates in 2 of 3 yr, although those studies used different varieties. Crop coefficient functions using both short and tall standardized reference ET values are presented for sprinkler and subsurface drip irrigation systems. Although these data are useful to producers in the SHP, additional data are needed to develop Kc functions that better characterize interannual variations in weather.

DETERMINATION OF ACTUAL CROP EVAPOTRANSPIRATION (ETC) AND DUAL CROP COEFFICIENTS (KC) FOR COTTON, WHEAT AND MAIZE IN FERGANA VALLEY: INTEGRATION OF THE FAO-56 APPROACH AND BUDGET

Determination of the actual crop evapotranspiration (ETc) during the growing period is important for accurate irrigation scheduling in arid and semi-arid regions. Development of a crop coefficient (Kc) can enhance ETc estimations in relation to specific crop phenological development. This research was conducted to determine ETc values as well as daily and growth-stage-specific Kc for cotton, winter wheat and maize for silage at fields in Fergana Valley (Uzbekistan). The soil water balance model - BUDGET with integration of the dual crop procedure of the FAO - 56 was used to estimate the ETc and separate it into evaporation (Ec) and transpiration (Tc) components. An empirical equation was developed to determine the daily Kc values based on the estimated Ec and Tc. The Kc determination and comparison to existing FAO Kc values were performed based on 10, 5 and 6 study cases for cotton, wheat and maize, respectively. Mean seasonal amounts of crop water requirement in terms of ETc were 560 ± 50, 509 ± 27 and 243 ± 39 mm for cotton, wheat and maize, respectively. Estimated ETc for these crops were 1.10 - fold, 1.09 - fold and 0.73 - fold of recommended irrigation norm according to currently used hydromodule zoning (GMR) under semi-hydromorphic reclamation regime in Fergana province. The growth-stage-specific Kc for cotton, wheat and maize was 0.15, 0.27 and 0.11 at initial; 1.15, 1.03 and 0.56 at mid; and 0.45, 0.89 and 0.53 at late season stages. These Kc values correspond to those reported by the FAO - 56. Development of site specific Kc helps tremendously in irrigation management and furthermore provides precise water applications in the region. The developed simple approach to estimate daily Kc for the three main crops grown in the Fergana region was a first attempt to meet this issue.

Comparison of Lysimeter-Derived Crop Coefficients for Legacy and Modern Drought-Tolerant Maize Hybrids in the Texas High Plains

HighlightsDaily maximum crop coefficient (Kc) values were similar for legacy hybrids and a modern drought-tolerant (DT) hybrid.Later planting dates for a DT hybrid resulted in average season lengths ~25 days shorter than those of legacy hybrids.Results illustrated the effects of environment, planting date, interannual variation in temperature, and the importance of climate-specific Kc functions.DT hybrids may be more effective at reproductive growth during periods of heat stress in semi-arid environments, although additional data are needed to support this conclusion.Abstract. Corn (Zea mays L.) is a major irrigated crop grown in the Southern High Plains including the Texas Panhandle. Irrigation from the Ogallala Aquifer is required to sustain profitable corn production in the region by supplementing inadequate and erratic rainfall. Effective irrigation scheduling works to extend limited groundwater resources by avoiding water losses associated with runoff and/or percolation below the root zone. The use of crop coefficient (Kc) and reference evapotranspiration (ETo) values to estimate daily crop water use (ETc) remains an effective scheduling tool that can complement other irrigation scheduling approaches. Both Food and Agriculture Organization (FAO-56) piecewise and curvilinear Kc values for corn are found in the literature. However, advances in corn genetics have led to questions about the applicability of Kc values developed using legacy corn hybrids to irrigation of modern drought-tolerant (DT) hybrids. Lysimeter-derived Kc values for legacy corn hybrids grown in large weighing lysimeter fields at the USDA-ARS Conservation and Production Research Laboratory at Bushland, Texas, were compared with those derived from a modern DT corn hybrid recently grown in the same fields. Results indicated that although midseason daily Kc values were similar for all hybrids, average season length was ~25 days shorter for the modern DT hybrid, characterized by a shortened initial growth period followed by more rapid increase of Kc during the development period. However, plots of Kc over thermal time illustrated that the differences in season length were likely attributable to later planting dates associated with the DT corn hybrids. Average seasonal water use was 730 and 811 mm for the legacy and modern DT hybrids, respectively (three years each), with corresponding average yields of 1.2 and 1.4 kg ha-1. Results suggest that published Kc and Kcb values developed with legacy corn hybrids remain largely applicable to modern DT corn hybrids when used with accurate estimates of effective canopy-based growth stages and climate-specific Kc functions. Keywords: Crop coefficients, Drought-tolerant, Evapotranspiration, Maize, Weighing lysimeters.

Evapotranspiration and crop coefficient patterns of an apple orchard in a sub-humid environment

Increasing water use efficiency is one of the main challenges of sustainable fruit tree production. From 2013 to 2015 we measured actual evapotranspiration (ETa) using eddy covariance in a well-irrigated apple orchard located in in South Tyrol (Italy), a sub-humid environment. We assessed the experimental crop coefficient (Kcexp) and analyzed the dependency of Kc on specific environmental variables at a daily time scale. Kcexp values changed throughout the season following a bell-shaped trend and were generally lower than the FAO tabular values corrected for local climatic conditions. In the mid-season phase, when LAI and tabular Kc are supposed to be constant, the average experimental Kc (Kc¯exp) was 1.01, 86% of the Kc value reported by FAO (1.18). Mid-season Kc residuals (Kcexp - Kc¯exp) were positively correlated with daily vapor pressure deficit (VPD) (ρ = 0.45), suggesting that the daily Kc variability observed is due, at least in part, to changes in the evaporative demands of the atmosphere. We explain these results by considering the relatively humid environment, the high water availability and the fact that leaves on apple trees are more tightly coupled to the atmosphere with respect to a smoother grass surface.

Relationship Between Spatio-Temporal Leaf Area Index and Crop Coefficient When Monitoring Coffee Plots in Brazil Using the Sentine-2

Robust monitoring techniques for perennial crops have become increasingly possible due to technological advances in the area of Remote Sensing (RS), and the products are available through the European Space Agency (ESA) initiative. RS data provides valuable opportunities for detailed assessments of crop conditions at plot level using high spatial, spectral, and temporal resolution. This study addresses the monitoring of coffee at the plot level using RS, analyzing the relationship between the spatio-temporal variability of the Leaf Area Index (LAI) and the crop coefficient (Kc); the Kc being a biophysical variable that integrates the potential hydrological characteristics of an agroecosystem compared to the reference crop. Daily and one-year Kc were estimated using the relation of crop evapotranspiration and reference. ESA Sentinel-2 images were pre-analyzed and atmospherically corrected, and Top-of-the-Atmosphere (TOA) reflections converted to Top-of-the-Canopy (TOC) reflectance. The TOCs resampled at the 10m resolution, and with the angles corresponding to the directional information at the time of the acquisition, the LAI was estimated using the trained neural network available in the Sentinel Application Platform (SNAP). During 75% of the monitored days, Kc ranged between 1.2 and 1.3 and, the LAI analyzed showed high spatial and temporal variability at the plot level. Based on the relationship between the biophysical variables, the LAI variable can substitute the Kc and be used to monitor the water conditions at the production area as well as analyze spatial variability inside that area. Sentinel-2 products could be more useful in monitoring coffee in the farm production area.

The characteristics of evapotranspiration and crop coefficients of an irrigated vineyard in arid Northwest China

Oasis effect may have significant impacts on the crop water use in irrigated fields of extremely arid conditions. Studies on crop coefficients and its relevant influence factors are essential to accurately determine the irrigation scheduling under such conditions. A two-year experiment was conducted in an irrigated vineyard to estimate evapotranspiration (ET) and its two components: evaporation (E) and crop transpiration (T), and to develop appropriate region-specific crop coefficients (Kc, Kcb) in arid Northwest China. The contributions of “oasis effect” to Kc, the relationships of both Kc and Kcb to canopy conductance (Gc) and leaf area index (LAI) were investigated. Local values of Kc for evapotranspiration and crop transpiration (Kcb) were calculated using the field data and the dual-Kc approach. The daily ET ranged from 0.80 to 9.87 mm d−1 and from 0.99 to 8.73 mm d−1 in 2014 and 2015, respectively, with corresponding T/ET values amounting to approximately 58.4% and 55.1%. The locally averaged Kc values were 0.79, 1.31, and 1.08 during the initial, middle, and late growth stages, respectively, versus corresponding Kcb values of 0.17, 0.97, and 0.64. Arid advection accounted for 16.9 to 57.4% and from 1.4 to 49.0% of daily Kc during the two study periods. Both Kc and Kcb to LAI is a linear regression but to Gc is an exponential one. LAI is a better indicator than Gc when it is used to predict Kc and Kcb. These results will help growers to improve irrigation efficiency and quantify the contributions of individual factors to Kc and Kcb in such conditions.

Evapotranspiration and crop coefficient (Kc) of pre-sprouted sugarcane plantlets for greenhouse irrigation management

The sugarcane pre-sprouted plantlets (PSP) production system is an innovative method to enhance crop multiplication rates. This system is greenhouse-grown, where correct irrigation management is essential to maintain the production quality. The purpose of this study was to determine the water requirement of three cultivars (CTC9005HP, RB966928, and SP87365) of pre-sprouted sugarcane plantlets, in which crop evapotranspiration (ETc) is a function of the product of the crop coefficient (Kc) and reference evapotranspiration (ETo). Three specifically built PSP-weighing lysimeters were used to determine ETc, while ETo was calculated by the FAO-56 method, making it possible to assess Kc values during the PSP cycle production. The ETc ranged from 3 to 6.9 mm d−1 for CTC9005HP, 3.1 to 6.8 mm d−1 for RB966928, and 2.9 to 6.6 mm d−1 for SP87365. Daily Kc values increased from 1.00 to 1.46 for CTC9005HP, 1.02 to 1.53 for RB966928, and 1.02 to 1.49 for SP87365. This research makes evident the importance of assessing appropriate Kc values in order to estimate crop water requirements and irrigation needs. Therefore, these results are appropriate for proper irrigation management in a greenhouse-grown sugarcane PSP.

A lysimeter study for the effects of different canopy sizes on evapotranspiration and crop coefficient of summer maize

Canopy size has an important and direct influence on water consumption by raising maize (Zea mays L.) transpiration through increased stomata number on leaves and reducing soil evaporation through blocking solar radiation. Evapotranspiration of maize was determined between 2012 and 2013 by weighing lysimeters at National experimental station for precise agriculture in Beijing. The results showed that total water consumption was 386.3 mm and 366.2 mm for maize strain with large (L) and small canopy (S), respectively. From DAS 50 to DAS 80, the difference of water consumption between two maize strains enlarged at a higher rate than other periods, which was about 4.2 mm at intervals of 10 days. Both the highest periodical water consumption and daily water consumption occurred at the Mid-stage, which was 151.8 mm for L and 139.1 mm for S, 5.06 mm/day for L and 4.64 mm/day for S, respectively. The crop coefficient was 1.11 and 1.04 and seasonal crop coefficients at Initial stage, Mid-stage and Late stage were 0.46, 1.53, 1.22 and 0.44, 1.40, 1.09 for L and S, respectively. The duration when daily Kc was bigger than 1(Kc > 1) was 56 days for L and was 5 days more than that of S, respectively. Although bigger canopy consumed more water, a higher water use efficiency of L is not only for its higher averaged yield, but also for its more positive response of yield to ET increment than that of S between two years. The stable yield performance is helpful for a long-term high water use efficiency. The characteristics of water consumption for maize of different canopy size may help to optimize water utilization in semiarid region.

Variation of the photosynthetic electron transfer rate and electron requirement for daily net carbon fixation in Ariake Bay, Japan

Fast repetition rate fluorometry (FRRf) provides a potential means to examine marine primary productivity; however, FRRf-based productivity estimations require knowledge of the electron requirement (K) for carbon (C) uptake (K C) to scale an electron transfer rate (ETR) to the CO2 uptake rate. Most previous studies have derived K C from parallel measurements of ETR and CO2 uptake over relatively short incubations, with few from longer-term daily-integrated periods. Here we determined K C by comparing depth-specific, daily ETRs and CO2-uptake rates obtained from 24-h on-deck incubation experiments undertaken on seven cruises in Ariake Bay, Japan, from 2008 to 2010. The purpose of this study was to determine the extent of variability of K C and to what extent this variability could be reconciled with the prevailing environmental conditions and ultimately to develop a method for determining net primary productivity (NPP) based on FRRf measurements. Both daily ETR and K C of the upper layer varied considerably, from 0.5 to 115.7 mmol e− mg Chl-a −1 day−1 and 4.1–26.6 mol e− (mol C)−1, respectively, throughout the entire data set. Multivariate analysis revealed a strong correlation between daily photosynthetically active radiation (PAR) and K C (r 2 = 0.94). A simple PAR-dependent relationship derived from the data set was used for generating K C, and this relationship was validated by comparing the FRRf-predicted NPP with the 13C uptake measured in 2007. These new observations demonstrate the potential application of FRRf for estimating regional NPP from ETR.

Evapotranspiration measurement and crop coefficient estimation over a spring wheat Farmland ecosystem in the Loess Plateau.

Evapotranspiration (ET) is an important component of the surface energy balance and hydrological cycle. In this study, the eddy covariance technique was used to measure ET of the semi-arid farmland ecosystem in the Loess Plateau during 2010 growing season (April to September). The characteristics and environmental regulations of ET and crop coefficient (Kc) were investigated. The results showed that the diurnal variation of latent heat flux (LE) was similar to single-peak shape for each month, with the largest peak value of LE occurring in August (151.4 W m−2). The daily ET rate of the semi-arid farmland in the Loess Plateau also showed clear seasonal variation, with the maximum daily ET rate of 4.69 mm day−1. Cumulative ET during 2010 growing season was 252.4 mm, and lower than precipitation. Radiation was the main driver of farmland ET in the Loess Plateau, which explained 88% of the variances in daily ET (p<0.001). The farmland Kc values showed the obvious seasonal fluctuation, with the average of 0.46. The correlation analysis between daily Kc and its major environmental factors indicated that wind speed (Ws), relative humidity (RH), soil water content (SWC), and atmospheric vapor pressure deficit (VPD) were the major environmental regulations of daily Kc. The regression analysis results showed that Kc exponentially decreased with Ws increase, an exponentially increased with RH, SWC increase, and a linearly decreased with VPD increase. An experiential Kc model for the semi-arid farmland in the Loess Plateau, driven by Ws, RH, SWC and VPD, was developed, showing a good consistency between the simulated and the measured Kc values.

Crop coefficient and evapotranspiration of grain maize modified by planting density in an arid region of northwest China

In order to investigate crop coefficient (Kc) and evapotranspiration (ET) of grain maize modified by planting density, a field experiment was conducted from March to October in 2012 and 2013 in an arid region of northwest China. Five planting densities, i.e. 67,500, 82,500, 97,500, 112,500 and 127,500plantsha−1 were conducted in 2012, and a higher planting density of 142,500plantsha−1 was added in 2013. We introduced a density ratio (Kdensity) that is a function of leaf area index (LAI) to account for the effect of planting density on Kc, and the daily Kc can be computed by Kdensity multiplying Kc at the reference planting density (127,500plantsha−1). The Allen method considering an adjustment coefficient (Acm), the single and dual Kc methods considering a density coefficient (Kd), and Kdensity method were used to calculate Kc, and then the ET estimated by reference evapotranspiration and Kc was validated by the measured ET using the eddy covariance system in 2012 and 2013. Results showed that higher planting density increased ET and Kc and lowered soil evaporation and evaporation coefficient within the planting densities of the experiments. Daily ET estimated by the Allen method performed very well after the end of maize development stage, with mean bias error (MBE) of −0.06 and 0.12mmd−1, root mean square error (RMSE) of 0.84 and 0.80mmd−1 in 2012 and 2013 respectively. The single and dual Kc methods can better simulate the daily ET when actual LAI was below the maximum LAI. Compared to the three above-mentioned methods, the Kdensity method had higher accuracy in estimating daily ET over the whole stage, with higher R2 and lower MBE and RMSE, indicating that Kdensity method had better performance in calculating daily ET under different planting densities of grain maize.

Variations of crop coefficient and its influencing factors in an arid advective cropland of northwest China

Understanding the variation and magnitude of crop coefficient (Kc) is important for accurate determination of crop evapotranspiration and water use. In this study, we calculated Kc in an irrigated maize field with ground mulching by eddy covariance evapotranspiration measurements during the whole growing periods in 2009 and 2010 in an arid region of northwest China. A semi-empirical practical approach for estimating Kc was proposed by introducing the dynamic fraction of canopy cover and incorporating the effect of leaf senescence as a function of days after sowing. The contribution of arid advection of sensible heat resulting from irrigation to Kc and the response of Kc to canopy conductance (Gc) were investigated. The averaged values of daily Kc were lower than typical values obtained previously without mulching due to decreasing effect of mulching on Kc, with 0.82 and 0.80 for the 2 years, respectively. The maximum average Kc occurred at the heading stage, with 1.21 and 1.04 for the 2 years, respectively. The difference of Kc was attributed to the difference of leaf area index. The semi-empirical practical approach could well estimate the variations of Kc, thus could be a robust and useful tool for the practical users and water managers. The contributions to daily Kc from the arid advection were 4.4–28.0% of the measured Kc. The Gc had stronger control on daily Kc at the early and later stages than at the middle stage. When Gc, leaf area index and relative soil extractable water were lower than the respective threshold values of 20 mm s−1, 3.0 m2 m−2 and 0.5, the daily Kc increased significantly with the increase of the three factors, and almost remained constant when the three factors were beyond the threshold values. These results are helpful for quantifying contributions of individual factors to Kc, and subsequently improving water management practices according to Kc. Copyright © 2014 John Wiley & Sons, Ltd.

Characteristics and modeling of evapotranspiration over a temperate desert steppe in Inner Mongolia, China

The magnitude and seasonal dynamics of evapotranspiration (ET) over a temperate desert steppe in Inner Mongolia, China, were investigated during a growing season (from 1st May to 15th October 2008) using the eddy covariance (EC) technique. Diurnal variations of latent heat flux (LE) at different phenological stages showed similar patterns with the peak value around noon. The sensible heat flux dominated the latent heat flux at midday during the whole growing season. Seasonal variation in ET followed closely the variation in soil water content (SWC) and leaf area index (LAI). The maximum daily ET rate was 3.8 mm day−1. Cumulative ET during the study period estimated directly by EC method was 150.9 mm, exceeding the precipitation (PPT) received at the site during the same period (133.0 mm) by 13.4%. The daily crop coefficient (Kc, calculated as the ratio of the measured ET (from the EC system) to the reference ET (from the Penman–Monteith model)) also showed a distinct seasonal pattern, however its value on average was considerable lower (0.17) during the measurement period than the results reported from other grasslands, suggesting that this steppe was strongly affected by the deficit of available water. SWC was the most important environmental factor controlling ET in this steppe. The correlation analysis between daily Kc and its major environmental controls indicated that SWC, relative humidity (RH), and net radiation (Rn) were the major explanatory variables for daily Kc. Daily Kc showed a linear increase with SWC and Rn, and an exponential increase with RH. Thus, a daily Kc model would be developed as a function of SWC, RH, and Rn. An empirical daily ET model derived from the FAO 56 crop coefficient approach was developed based on this daily Kc model and the Penman–Monteith model. This ET model was validated by the data from the growing season in 2009, and showed a good consistency between the simulated and the measured ET values. Based on the developed ET model, an alternative ET model with the variable of SWC replaced by PPT was established, which may have the advantage of ET estimation under PPT more easily available than SWC.

A comparative study of apple and pear tree water consumption measured with two weighing lysimeters

A five-year experiment (2002–2006) was conducted to determine apple (cv ‘Golden Smoothee’) and pear (cv ‘Conference’) crop coefficients (Kc) using two large weighing lysimeters. Daily reference evapotranspiration (ETo) and crop evapotranspiration (ETc) were evaluated. Midday canopy light interception of both crops planted in hedgerows, 4 × 1.6 m, was determined on a weekly basis from bud-break until leaf fall from year 2002 (fourth after planting) to year 2006 of both plantations. Relationships between canopy light interception and calculated Kc (ETc/ETo) were evaluated from bud-break until harvest. There were differences in Kc values between apple and pear trees. When daily Kc values from bud-break until harvest were adjusted to hyperbolic functions each year, adjusted curves for pear trees were very similar regardless of year (maximum Kc around 1.0). In apple trees, the maximum values of Kc increased over time, from 0.49 in 2002 to 1.04 in 2006. Midday light interception in both apple and pear trees increased during the 5 years of experiment from 29.0 to 45.6% in apples and from 27.5 to 41.6% in pears in midsummer. Although there was a significant positive correlation between midday canopy light interception and Kc in apple and pear trees, in different times within a specific year, these relationships were different between crops. While the apple data fitted into the same equation regardless of the year, different equations were needed to fit the pear data in different years. This discrepancy may have been related to differences in the canopy properties between apple and pear trees. Pear canopies had higher porosity than apple canopies and thus improved light penetration. Apple trees were more vigorous and produced taller and denser canopies. Pear Kc values were greatly influenced by the evaporative demands of different years and consequently differences in midday canopy light interception did not adequately reflect the differences in Kc across the two species.

Bahiagrass crop coefficients from eddy correlation measurements in central Florida

Bahiagrass (Paspalum notatum) is a warm-season grass used primarily in pastures and along highways and other low maintenance public areas in Florida. It is also used in landscapes to some extent because of its drought tolerance. Bahiagrass can survive under a range of moisture conditions from no irrigation to very wet conditions. Its well-watered consumptive use has not been reported previously. In this study, bahiagrass crop coefficients (Kc) for an irrigated pasture were determined for July 2003 through December 2006 in central Florida. The eddy correlation method was used to estimate crop evapotranspiration (ETc) rates. The standardized reference evapotranspiration (ETo) equation (ASCE-EWRI standardization of reference evapotranspiration task committee report, 2005) was applied to calculate ETo values using on site weather data. Daily Kc values were estimated from the ratio of the measured ETc and the calculated ETo. The recommended Kc values for bahiagrass are 0.35 for January–February, 0.55 for March, 0.80 for April, 0.90 for May, 0.75 for June, 0.70 for July–August, 0.75 for September, 0.70 for October, 0.60 for November, and 0.45 for December in central Florida. The highest Kc value of 0.9 in May corresponded with maximum vapor pressure deficit conditions as well as cloud free conditions and the highest incoming solar radiation as compared to the rest of the year. During the summer (June to August), frequent precipitation events increased the cloud cover and reduced grass water use. The Kc annual trend was similar to estimated Kc values from another well-watered warm-season grass study in Florida.

Evapotranspiration rates and crop coefficients for a restored marsh in the Sacramento–San Joaquin Delta, California, USA

AbstractThe surface renewal method was used to estimate evapotranspiration (ET) for a restored marsh on Twitchell Island in the Sacramento–San Joaquin Delta, California, USA. ET estimates for the marsh, together with reference ET measurements from a nearby climate station, were used to determine crop coefficients over a 3‐year period during the growing season. The mean ET rate for the study period was 6 mm day−1, which is high compared with other marshes with similar vegetation. High ET rates at the marsh may be due to the windy, semi‐arid Mediterranean climate of the region, and the permanently flooded nature of the marsh, which results in very low surface resistance of the vegetation. Crop coefficient (Kc) values for the marsh ranged from 0·73 to 1·18. The mean Kc value over the entire study period was 0·95. The daily Kc values for any given month varied from year to year, and the standard deviation of daily Kc values varied between months. Although several climate variables were undoubtedly responsible for this variation, our analysis revealed that wind direction and the temperature of standing water in the wetland were of particular importance in determining ET rates and Kc values. Published in 2007 by John Wiley &amp; Sons, Ltd.

ALFALFA BASAL CROP COEFFICIENTS FOR FAO–56 PROCEDURES IN THE DESERT REGIONS OF THE SOUTHWESTERN U.S.

The FAO Irrigation and Drainage Paper No. 56 (FAO56) is expected to provide a universally consistentmethodology for obtaining reliable estimates of crop evapotranspiration (ETc) from standard weather data and cropcoefficient (Kc) information. Yet, for many areas, including the southwestern desert region of the U.S., evaluation of theFAO56 crop coefficient methods for local conditions has been limited. In this study, measurements of daily alfalfa ETc wereused to evaluate the dual crop coefficient approach of FAO56, which separates the single Kc into two coefficients, a basalcoefficient, Kcb (primarily transpiration), and a wet soil evaporation coefficient, Ke. A second objective was to calibratealfalfa Kcb baselines for constructing localized, climateadjusted FAO56 Kcb curves for growth cycles under the seasonalclimatic variations encountered in the semiarid, southwestern U.S. Daily ETc was measured for alfalfa in three weighinglysimeters subjected to varying irrigation regimes for eight cutting cycles in 1985 at a field site in Phoenix, Arizona. DailyKc data were determined from the measured ETc and calculations of the FAO56 grassreference evapotranspiration (ETo)using meteorological data obtained at the site. Daily Kc values were partitioned into the dual crop coefficients (Kcb and Ke)using backcalculations of the FAO56 dual procedures. Examples are given to illustrate how the magnitudes of Kcb, Ke, andKc varied with respect to crop development and wet and dry surface soil conditions. The effects of water stress on ETc wereconsidered in the backcalculations using the FAO56 water stress reduction coefficient (Ks). Linearstyle FAO56 Kcbcurves were then constructed separately for each lysimeter for each of the eight cutting cycles based on the backcalculatedKcb data. Daily ETc based on the constructed curves and FAO56 calculation procedures were highly correlated to the dailymeasured lysimeter ETc (r2 = 0.98), and the mean daily difference between calculated and measured ETc (0.03 mm) was notsignificant (p > 0.7). Comparison of measured ETc for waterstressed and wellwatered alfalfa indicated that calculated Ksdescribed ETc reductions due to soil water stress adequately when atmospheric evaporative demand was considered in theKs calculation. Baseline Kcb values for constructing local climateadjusted FAO56 alfalfa Kcb curves were determined as0.30, 1.22, and 1.05 for the initial, midseason, and end of late season, respectively.