Dual Crop Coefficient Research Articles

A sensitivity analysis of a FAO-56 dual crop coefficient-based model under various field conditions

FAO-56 dual crop coefficient (FAO-2Kc) based model are increasingly applied at large scale for agricultural water monitoring, requiring field-scale data over the spatial extent of interest. Given the lack of in-situ measurements, satellite products can be used to estimate indirectly the parameters through calibration. However, a lack of knowledge about model sensitivity can lead to suboptimal use of satellite data. This study aims to analyze the sensitivity of SAMIR, a FAO-2Kc-based model using satellite data. The Sobol method was applied for evapotranspiration (ET) and deep percolation (DP) simulations on 37 contrasted agricultural seasons. Results indicate that SAMIR's sensitivity mainly depends on the modeled water stress. We proposed a proxy for the model sensitivity which can determine 84% (73%) of the ET (DP) among the agricultural seasons. An interaction analysis allowed reducing the calibration problem to the adjustment of only two parameters (a_Kcb and Zr_max), accounting for most of the sensitivity.

A novel method for simulating the dynamics of the single and dual maize crop coefficients in an arid ecosystem

The crop coefficient is a key link between crop evapotranspiration (ETc) and reference evapotranspiration (ET0), which is important for agricultural water management and irrigation strategy. However, how to quantify seasonal variations in the single and dual crop coefficients (Kc and Kcb, respectively) remains a challenge. This study aimed to provide a novel approach with a soil-plant-atmosphere continuum (SPAC) model under various land surface scenarios for reconstructing the dynamics of Kc and Kcb. A numerical SPAC model was validated by comparing with measurements both at hourly and daily mean dataset (e.g., eddy covariance for actual ET). The proposed model was then applied to the Heihe River Basin to calculate the seasonal and annual variations in both Kc and Kcb of maize. Kc and Kcb shown significant seasonality and interannual fluctuations. The Kc and Kcb presented a single‐peak trend, reaching the maximum value in peaking growing season and fluctuating day to day. During growing season, changes in Kc, Kcb and the coefficient of soil evaporation (Ke) varied from 0.19 to 1.57, 0.08–1.50, and 0.01–0.19, with average values of 0.35, 0.39, and 0.04, respectively. It was found that leaf area index (LAI) was the dominant factor regulating Kc, Kcb, and Ke. This study implied that the SPAC model with numerical technique is efficient in reconstructing the dynamics (annual and inter-annual) of the crop coefficient and feasible for diagnosing complex interactions among soil, plants, and the atmosphere under various land surface scenarios.

Evaluating maize evapotranspiration using high-resolution UAV-based imagery and FAO-56 dual crop coefficient approach

Timely and accurate estimation of crop evapotranspiration (ETc) is essential for efficient irrigation management at the farmland scale. However, effective decision-making for irrigation scheduling requires high spatiotemporal-resolution data to provide within-field heterogeneity information. The objective of this study was to evaluate the use of optical and thermal information obtained from an unmanned aerial vehicle (UAV) to quantify maize (Zea mays L.) ETc within the framework of the FAO-56 dual crop coefficient approach. Canopy temperature-based crop water stress index (CWSI) and number of degrees above canopy threshold (DACT) were used to determine the stress coefficient (Ks). Three types of normalized difference vegetation index (NDVI), soil adjusted vegetation index (SAVI), and enhanced vegetation index (EVI) were adopted to determine basal crop coefficient (Kcb). Then the two forms of stress coefficient in combination with three vegetation indices (VIs) were evaluated to estimate daily crop ETc under different irrigation treatments over two years in the southwest region of Inner Mongolia, China. The results demonstrated that the combination of NDVI and CWSI produced the best estimates of maize ETc, with R2 of 0.84 and root mean square error (RMSE) of 0.50 mm/day. The DACT-based model also performed well, with R2 of 0.77–0.80 and RMSE of 0.53 mm/day. Although the results varied with irrigation levels, the daily mean bias error of ETc predictions over different years indicated acceptable accuracy, with a mean bias error (MBE) of 0.48 mm/day. Sensitivity analyses indicated that ETc models were most sensitive to the slope of the linear regression between Kcb and VIs, followed by soil evaporation constant and influential factor in Ks_DACT. From this study, the combinations of vegetation index and CWSI, as well as DACT, were recommended as alternative approaches for estimating ETc due to intrinsic simplicity and easy interpretation. These ETc models relying on UAV-based multi-sensor data thus show promising potential in farmland-scale applications.

Comparison of Shuttleworth–Wallace and Dual Crop Coefficient Method for Estimating Evapotranspiration of a Tea Field in Southeast China

Determination of evaporation (E) and transpiration (T) in tea fields separately is important in developing precise irrigation scheduling and enhancing water use efficiency. In this study, the Shuttleworth–Wallace (S-W) model was applied to simulate the variations of E and T based on the data from 2015 to 2018 in a tea field in southeast China. The dual crop coefficient (D-K) method recommended by FAO-56 was also applied to calculate E and T, using the same data set to compare with the S-W model. The measured crop coefficient (Kc) ranged from 0.43 to 1.44 with the average value was 0.90 during 1–150 DOY (days of year), and the measured Kc tended to be stable with the average value of 0.83 during 151–365 DOY in 2015. The S-W model estimated ETc with root mean square error (RMSE) and R2 of 0.45 mm d−1 and 0.97, while for the D-K method the values were 0.61 mm d−1 and 0.95. Therefore, both approaches could estimate the E and T separately in tea fields in southeast China, however, the D-K method had a slightly poorer accuracy compared to the S-W model in the estimation of ETc.

Misconceptions of Reference and Potential Evapotranspiration: A PRISMA-Guided Comprehensive Review

One of the most important parts of the hydrological cycle is evapotranspiration (ET). Accurate estimates of ET in irrigated regions are critical to the planning, control, and regulation of agricultural natural resources. Accurate ET estimation is necessary for agricultural irrigation scheduling. ET is a nonlinear and complex process that cannot be calculated directly. Reference evapotranspiration (RET) and potential evapotranspiration (PET) are two primary forms of ET. The ideas, equations, and application areas for PET and RET are different. These two terms have been confused and used interchangeably by researchers. Therefore, terminology clarification is necessary to ensure their proper use. The research indicates that PET and RET concepts have a long and distinguished history. Thornthwaite devised the original PET idea, and it has been used ever since, although with several improvements. The development of RET, although initially confused with that of PET, was formally defined as a standard method. In this study, the Preferred Reporting Item for Systematic reviews and Meta-Analysis (PRISMA) was used. Equations for RET estimation were retrieved from 44 research articles, and equations for PET estimation were collected from 26 studies. Both the PET and RET equations were divided into three distinct categories: temperature-based, radiation-based, and combination-based. The results show that, among temperature-based equations for PET, Thornthwaite’s (1948) equation was mentioned in 12,117 publications, whereas among temperature-based equations for RET, Hargreaves and Samani’s (1985) equation was quoted in 3859 studies. Similarly, Priestley (1972) had the most highly cited equation in radiation-based PET equations (about 6379), whereas Ritchie (1972) had the most highly cited RET equations (around 2382) in radiation-based equations. Additionally, among combination-based PET equations, Penman and Monteith’s (1948) equations were cited in 9307 research studies, but the equations of Allen et al. (1998) were the subject of a significant number of citations from 23,000 publications. Based on application, PET is most often applied in the fields of hydrology, meteorology, and climatology, whereas RET is more frequently utilized in the fields of agronomy, agriculture, irrigation, and ecology. PET has been used to derive drought indices, whereas RET has been employed for single crop and dual crop coefficient approaches. This work examines and describes the ideas and methodologies, widely used equations, applications, and advanced approaches associated with PET and RET, and discusses future enhancements to increase the accuracy of ET calculation to attain accurate agricultural irrigation scheduling. The use of advanced tools such as remote sensing and satellite technologies, in addition to machine learning algorithms, will help to improve the accuracy of PET and RET estimates. Researchers will be able to distinguish between PET and RET in the future with the use of the study’s results.

Changes and determining factors of crop evapotranspiration derived from satellite-based dual crop coefficients in North China Plain

Evaluating actual crop evapotranspiration (ETc) variations and their determining factors under changing climates is crucial for agricultural irrigation management and crop productivity improvement in non-humid regions. This study analyzed the spatiotemporal characteristics and detected the determining factors of ETc for winter wheat and summer maize rotation system from 2000 to 2017 in the North China Plain (NCP), by combining the FAO-56 dual crop coefficient approach with remotely sensed vegetation indices (VIs). The results indicated that daily air temperature increased in varying degrees while wind speed and sunshine hours decreased slightly during the growing season of winter wheat and summer maize over the study period. The trends of relative humidity and effective precipitation varied in crop growing seasons. Based on the validated relationship of dual crop coefficients and VIs, the estimated multi-year average ETc of winter wheat (370.29 ± 31.28 mm) was much higher than summer maize (281.85 ± 20.14 mm), and the rotation cycle was 652.43 ± 27.67 mm. Annual ETc of winter wheat and the rotation cycle increased by 2.96 mm a−1 and 1.77 mm a−1, respectively. However, the ETc of summer maize decreased with distinct spatial variation. Spatially, winter wheat ETc increased significantly in the northeast NCP, covering the Beijing-Tianjin-Hebei areas. Meanwhile, significant increases in summer maize ETc were detected in the southwest NCP. The sensitivity and contribution analysis showed that ETc of winter wheat and summer maize was positively sensitive to temperature, wind speed, and sunshine hours while negatively to relative humidity. Moreover, wind speed and sunshine hours contributed most to changes in ETc (around 20%–40%).

Influence of different levels of irrigation and black plastic mulch on the performance of banana under drip irrigation

India, a leading fruit producing country, ranks first in banana cultivation. The crop is highly water demanding and that appears as a limitation in several areas. However, micro-irrigation has the potential to address the issue. Moreover, mulching with black plastic sheet further facilitates efficient water use. The combined application of micro-irrigation and black plastic mulch is potentially important for enhancing water use efficiency and crop productivity. Based on the above facts, a field trial was conducted to evaluate the performance of micro-irrigation levels and black plastic mulch on the performance of banana yield in the sub humid climatic condition in lateritic belt during 2019–2020. Modified penman-Montieth equation was used to estimate the crop water requirement considering the recorded local climatic data. Water requirement of banana under drip and plastic mulch may vary with non-mulched banana. Therefore, the dual crop coefficient approach has been used to estimate the crop coefficients (K) of banana under drip with black plastic mulch and without mulch conditions. The total water requirement of banana was estimated to be 1089 mm and 919 mm under non-mulched and black plastic mulched banana, respectively. To determine the optimum quantity of irrigation water requirement of banana, eight treatments were considered consisting of four irrigation levels, namely, 1.0 volume with drip (VD), 0.8 VD, 0.6 VD and 0.4 VD in combination with black plastic mulch and non-mulch. The results revealed that 0.8 VD along with black plastic mulch recorded better crop growth and yield parameters with higher yield in comparison to other treatments. Further, the treatment also resulted in maximum benefit cost ratio of 4.97.

Energy and evapotranspiration partitioning over a humid region orchard: Field measurements and partitioning model comparisons

Understanding energy and water flux cycling and quantifying their components are essential steps to elucidate the hydrological process and sustainable water resources management in agricultural ecosystems. The challenge of partitioning energy and evapotranspiration (ET) components (canopy transpiration Tc and surface evaporation Es) was addressed by the combination of eddy covariance (EC) technique, micro-lysimeters, and sap flow methods in a humid region kiwifruit orchard of Southwest China. The variations of energy and water flux and their controls were assessed, then the measured ET and its component were compared with the simulated values by Priestley-Taylor Jet Propulsion Laboratory model (PT-JPL), Shuttleworth-Wallace (S-W), and FAO-56 dual crop coefficient models (Dual-FAO). The results showed that latent heat flux (LE) dominated the consumption of net radiation energy (Rn), with the averaged ratio (EF = LE/Rn) above 0.70 during the growing seasons. More radiation energy is released in the form of LE with increased surface moisture and vegetation cover (total leaf area index, LAIt). Tc accounted for 41%–46% ET. The increase of canopy leaf area index (LAIc) strongly affected ET partitioning through the early growing seasons, while atmospheric evaporation demand dominated the Tc/ET and even caused the energy to be redistributed since high atmospheric evaporation demand caused a decrease of Tc while an increase of Es during the active growing seasons (LAIc > 2.5 m2 m−2). All the ET models performed well in estimating ET (R2 = 0.84–0.92) and its components (Tc, R2 = 0.70–0.74; Es, R2 = 0.63–0.69), allowing individual estimation of water interception by the canopy is one of the merits of the PT-JPL model for ET simulation in this humid site, with R2 of 0.92, RMSE of 0.55 mm d−1, while the S-W model performed slightly better in ET component simulations (Tc, R2 = 0.74, RMSE = 0.35 mm d−1; Es, R2 = 0.69, RMSE = 0.38 mm d−1). The results of our study may have much referential significance for eco-hydrological process analysis and water flux estimation in regions with similar environmental conditions.

Evapotranspiration Variations of the Minjiang River Basin in Southeastern China from 2000 to 2019

Evapotranspiration is one of the critical processes in the terrestrial hydrological cycle, and the assessment of evapotranspiration is essential for understanding the regional hydrological cycle. In this study, the Minjiang River Basin, a typical watershed in the humid subtropical climate zone, is selected as the study region. The Penman-Monteith equation and the dual crop coefficient method are used to calculate the actual evapotranspiration (ETa) at seven meteorological stations within the study basin. Meanwhile, the applicability of the Global Land Data Assimilation System-Noah (GLDAS-Noah) ETa data in the Minjiang River Basin is evaluated based on stations P-M equation results, then to analyze the changes of the ETa in the Minjiang River Basin from 2000 to 2019. The results show that the GLDAS-Noah ETa data are well applicable in the Minjiang River Basin (R2 > 0.9 and NSE > 0.8). The ETa in the basin shows an increasing trend since 2000, and the increasing rate is 3.60 mm·yr−1 (p < 0.01). The seasonal variation results show that ETa tends to increase in winter and spring, with increasing rates of 1.10 mm·yr−1 (p < 0.01) and 2.60 mm·yr−1 (p < 0.01), respectively, while the ETa did not change significantly in summer and autumn. Annual air temperature has the largest effect on annual ETa (59.6%), followed by precipitation at 33.9%. ETa increased in spring was mainly influenced by increasing temperatures (89.4%) in the Minjiang River Basin from 2000 to 2019. The research results are of great benefit to further improve the understanding of ETa variations in the basin under global warming.

Variation in actual corn (Zea mays L.) evapotranspiration, single, and dual crop coefficient under different point source irrigation systems in a semiarid region

Close management of irrigation could have considerable impacts on water resources, especially for cropping systems dominated by corn. The experiment was carried out to compare the influence of porous capsule irrigation (PCI), surface drip irrigation (DI), and subsurface drip irrigation (SDI) systems, with or without mulching, on actual evapotranspiration (ETc act), crop coefficients (Kc single and Kc dual), biomass yield, and water use efficiency (WUE) of corn in a semiarid region of Iran. The experiment was arranged in a split-plot design with the three irrigation systems assigned to the main plots and two mulching (with or without foil type) treatments (M1 and M2) assigned to the sub-plots. The corn ETc act varied significantly (P < 0.05) with the different irrigation systems, being (mm) 389.8 for PCI, 377.0 for DI, and 372.8 for SDI. The highest Kc average and Kcb (0.82 and 1.22) and the lowest Ke (0.12) were observed under the PCI system. The dry and wet biological biomass yields were highest (29.98 and 107 t/ha) under the PCI + M1 treatment, and the lowest (23.19 and 58.54 t/ha, respectively) were under the DI + M2 treatment. The highest WUE (7.89 kg/m3) was also observed under the PCI + M1 treatment; PCI produced the best biological biomass yield, WUE and IWUE in comparison to DI and SDI systems. Accordingly, the PCI system could be a viable alternative to drip irrigation for areas with scarce water resources, particularly for smallholder farmers.

Modeling Actual Water Use Under Different Irrigation Regimes at District Scale: Application to the FAO-56 Dual Crop Coefficient Method

Modeling Actual Water Use Under Different Irrigation Regimes at District Scale: Application to the FAO-56 Dual Crop Coefficient Method

Estimating and partitioning maize evapotranspiration as affected by salinity using weighing lysimeters and the SIMDualKc model

Water scarcity and saline stress are primary threats for water use and agricultural production in arid and semi-arid regions, such as the Hetao Irrigation District, China. The current study, using irrigation with saline water, developed through three seasons (2017–2019) of field experimental surveillance of mulched maize water use in two cropped weighing lysimeters. Measurements included crop characteristics (height, LAI, dates of development stages), soil and water salinity, and daily actual crop evapotranspiration (ETc act). Deficit surface irrigation was scheduled in 2017 and 2018 seasons, while in 2019 drip irrigation was used aimed at satisfying crop water requirements. Both approaches aimed at improving irrigation management while controlling impacts of salinity. The collected information was used to parameterize and calibrate the soil water balance model SIMDualKc. The model uses the FAO56 dual crop coefficient approach and considers the combined effect of water and salinity stresses due to salinity of both soil and irrigation water. Results show that the SIMDualKc model adequately simulates the dynamics of the observed ETc act throughout the three growing seasons. The goodness-of-fit indicators show highly appropriate model fitting of observed ETc act with low RMSE of 0.42 mm d-1 for the calibration and 0.53 mm d-1 for validation. The calibrated standard basal crop coefficients (Kcb) of maize for the initial, mid, and end stages were respectively 0.15, 1.15 and 0.25, which agree with those recently proposed by Pereira et al. (2021). Due to the impacts of both water and salt stress in 2017 and 2018, ETc act was much below the potential value ETc, ranging 64–83%, while in 2019 that percentage increased to 92% due to avoiding water stress by then. The average Kcb mid act range 0.58–1.06, therefore lower than Kcb mid, with the lower values occuring in the lysimeter that was irrigated with a large deficit. The irrigation scheduling practiced was assessed using water use and productivity indicators, which have shown the advantage in using drip irrigation with small time intervals between irrigations. Moreover, the current study and the calibrated model provide for adopting irrigation management practices that may save water and control salinity.

Yield and evapotranspiration characteristics of potato-legume intercropping simulated using a dual coefficient approach in a tropical highland

An accurate estimation of crop evapotranspiration and soil water balance under potato-legume intercropping is important for improving the crop water productivity of rainfed potato. This study quantified the evapotranspiration, yield, and soil water balance of potato (Solanum tuberosum L.) intercropped with lima bean (Phaseolus lunatus L.) or dolichos (Lablab purpureus L.) in comparison to the same crops under monocropping. The experiment was set up in three agroecological zones of Kenya: upper midland (with an altitude range of 1500–1653 m above sea level (masl)), lower highland (1892–1923 masl) and upper highland (2502–2594 masl). The dual crop coefficient approach was adopted with the SIMDualKc model to estimate crop evapotranspiration and compute the soil water balance. The dual crop coefficient partitions crop evapotranspiration into crop transpiration and soil evaporation by applying both soil evaporation and basal crop coefficient. The model was calibrated and validated using field data observed along four crop growth seasons. The basal crop coefficients, the ratios of soil evaporation to evapotranspiration, and that of transpiration to evapotranspiration were determined. The yields of different intercrops were converted into equivalent yield of potato based on price of the produce. Good agreement between the observed and simulated data for available soil water and crop evapotranspiration was found with modeling efficiency > 0.8. The residual mean square error was low and ranged from 0.01−0.08 m3 m-3 for available soil water and 0.03−0.09 mm d-1 for crop evapotranspiration. The transpiration to actual evapotranspiration ratio of potato-legume intercropping was 6−28% greater than that of the sole potato because legume intercrops fully covered the ground by mid-season, thus limiting the energy available for soil evaporation. Crop yields were significantly greater under intercropping as transpiration occurred near its potential rate, thus not limiting yields. These results support the dual crop coefficient method as an appropriate tool to estimate and accurately partition crop evapotranspiration under potato-legume intercropping.

Evapotranspiration Partition and Dual Crop Coefficients in Apple Orchard with Dwarf Stocks and Dense Planting in Arid Region, Aksu Oasis, Southern Xinjiang

Crop coefficients are critical to developing irrigation scheduling and improving agricultural water management in farmland ecosystems. Interest in dwarf cultivation with high density (DCHD) for apple production increases in Aksu oasis, southern Xinjiang. The lack of micro-irrigation scheduling limits apple yield and water productivity of the DCHD-cultivated orchard. A two-year experiment with the DCHD-cultivated apple (Malus × domestica ‘Royal Gala’) orchard was conducted to determine crop coefficients and evapotranspiration (ETa) with the SIMDualKc model, and to investigate apple yield and water productivity (WP) in response to different irrigation scheduling. The five levels of irrigation rate were designed as W1 of 13.5 mm, W2 of 18.0 mm, W3 of 22.5 mm, W4 of 27.0 mm, and W5 of 31.5 mm. The mean value of basal crop coefficient (Kcb) at the initial-, mid-, and late-season was 1.00, 1.30, and 0.89, respectively. The Kc-local (ETa/ET0) range for apple orchard with DCHD was 1.11–1.20, 1.33–1.43, and 1.09–1.22 at the initial, middle, and late season, respectively. ETa of apple orchard in this study ranged between 415.55–989.71 mm, and soil evaporation accounted for 13.85–29.97% of ETa. Relationships between total irrigation amount and apple yield and WP were developed, and W3 was suggested as an optimum irrigation schedule with an average apple yield of 30,540.8 kg/ha and WP of 4.45 kg/m3 in 2019–2020. The results have implications in developing irrigation schedules and improving water management for apple production in arid regions.

Evapotranspiration and Quantitative Partitioning of Spring Maize with Drip Irrigation under Mulch in an Arid Region of Northwestern China

To examine evapotranspiration (ETc), soil evaporation (Es), and transpiration (Tr), and partitioning of ETc, a two-year field experiment was carried out in a maize field with drip irrigation under mulch in an arid region of northwestern China in 2017 and 2018. In the experiment we designed two treatments with full irrigation (T1) and growth stage-based strategic regulated deficit irrigation (T2). The applied irrigation of T2 was 40% of the T1 during both late vegetative and reproductive growth stages. Based on the measurements of soil water content (SWC) and Tr, a dual crop coefficient model (SIMDualKc) was calibrated and validated, and daily ETc, Es, and Tr were estimated. The model can simulate well the dynamic variations of SWC and Tr. The calibrated basic crop coefficient at the initial, mid-season, and end growth stages was 0.2, 1.15, and 0.75, respectively. The ETc was 507.9 and 519.1 mm for the T1 treatment, and 428.9 and 430.9 mm for the T2 treatment. The ratios of Tr to ETc were higher for the two treatments, ~90%, for two years. Collectively, both drip irrigation under mulch and strategic deficit irrigation after canopy covering of the ground can significantly reduce the ineffective proportion of ETc and Es.

Assessing a Removable Mini-Lysimeter for Monitoring Crop Evapotranspiration Using a Well-Established Large Weighing Lysimeter: A Case Study for Barley and Potato

Further research is required on the measurement of crop evapotranspiration (ETc) to produce new or updated crop coefficients for a large number of crops using accurate weighing lysimeters. However, large weighing lysimeters are sometimes expensive and are not portable, and different prototypes of small-sized lysimeters may be a feasible alternative. This study evaluated the performance of a removable mini-lysimeter model to measure ETc and derive crop coefficients using a long-established large precision weighing lysimeter over a two-year period. The study was conducted during the 2017 and 2018 barley and potato growing seasons, respectively, at a lysimeter facility located in Albacete (southeast Spain). ETc values were determined using daily mass change in the lysimeters. Irrigation was managed to avoid any water stress. In the barley season, the mini-lysimeter underestimated the seasonal ETc by 2%, the resulting errors in barley ETc estimation were an MBE of −0.070 mm d−1 and an RMSE of ±0.289 mm d−1. In the potato season, the mini-lysimeter overestimated the cumulative ETc by 5%, the resulting errors in potato ETc measurement were an MBE of 0.222 mm d−1 and an RMSE of ±0.497 mm d−1. The goodness of fit indicators showed a good agreement between the large and mini-lysimeter barley and potato ETc measurements at daily time step. Single (Kc) and dual crop coefficients (Kcb, crop transpiration + Ke, soil evaporation) were derived from the lysimeter measurements, the grass reference evapotranspiration (ETo) and the FAO56 dual Kc approach; after temperate standard climate adjustment, mid-season values were Kc mid (std) = 1.05 and Kcb mid (std) = 1.00 for barley; and Kc mid (std) = 1.06 and Kcb mid (std) = 1.02 for potato. The good agreement found between Kcb values and fc will allow barley and potato water requirements to be accurately estimated.

Dual Crop Coefficient Approach in Vitis vinifera L. cv. Loureiro

Vineyard irrigation management in temperate zones requires knowledge of the crop water requirements, especially in the context of climate change. The main objective of this work was to estimate the crop evapotranspiration (ETc) of Vitis vinifera cv. Loureiro for local conditions, applying the dual crop coefficient approach. The study was carried out in a vineyard during two growing seasons (2019–2020). Three irrigation treatments, full irrigation (FI), deficit irrigation (DI), and rainfed (R), were considered. The ETc was estimated using the SIMDualKc model, which performs the soil water balance with the dual Kc approach. This balance was performed by calculating the basal coefficients for the grapevine (Kcb crop) and the active soil ground cover (Kcb gcover), which represent the transpiration component of ETc and the soil evaporation coefficient (Ke). The model was calibrated and validated by comparing the simulated soil water content (SWC) with the soil water content data measured with frequency domain reflectometry (FDR). A suitable adjustment between the simulated and observed SWC was obtained for the 2019 R strategy when the model was calibrated. As for the vine crop, the best fit was obtained for Kcb full ini = 0.33, Kcb full mid = 0.684, and Kcb full end = 0.54. In this sense, the irrigation schedule must adjust these coefficients to local conditions to achieve economically and environmentally sustainable production.

Transpiration and Water Use of an Irrigated Traditional Olive Grove with Sap-Flow Observations and the FAO56 Dual Crop Coefficient Approach

The SIMDualKc model was applied to evaluate the crop water use and the crop coefficient (Kc) of an irrigated olive grove (Olea europaea L.) located in Sicily, Italy, using experimental data collected from two crop seasons. The model applies the FAO56 dual Kc approach to compute the actual crop evapotranspiration (ETc act) and its components, i.e., the actual tree transpiration (Tc act), obtained through the basal crop coefficient (Kcb), and soil evaporation according to an evaporation coefficient (Ke). Model calibration was performed by minimizing the difference between the predicted Tc act and the observed daily tree transpiration measured with sap flow instrumentation (TSF field) acquired in 2009. The validation was performed using the independent data set of sap flow measurements from 2011. The calibrated Kcb was equal to 0.30 for the initial and non-growing season stages, 0.42 for the mid-season, and 0.37 for the end season. For both seasons, the goodness-of-fit indicators relative to comparing TSF field with the simulated Tc act resulted in root mean square errors (RMSE) lower than 0.27 mm d−1 and a slope of the linear regression close to 1.0 (0.94 ≤ b0 ≤ 1.00). The olive grove water balance simulated with SIMDualKc produced a ratio between soil evaporation (Es) and ETc act that averaged 39%. The ratio between actual (ETc act) and potential crop evapotranspiration (ETc) varied from 84% to about 99% in the mid-season, indicating that the values of ETc act are close to ETc, i.e., the adopted deficit irrigation led to limited water stress. The results confirm the suitability of the SIMDualKc model to apply the FAO56 dual Kc approach to tree crops, thus assessing the water use of olives and supporting the development of appropriate irrigation management tools that are usable by farmers. A different way to estimate Kcb is based on the approach suggested in 2009 by Allen and Pereira (A&amp;P), which involves the measured fraction of ground covered (shaded) by the crop and the height of the trees. Its application to the studied grove produced the mid-season Kcb values ranging from 0.40–0.45 and end-season Kcb values ranging from 0.35–0.40. The comparison between the A&amp;P-computed Tc act A&amp;P and TSF field shows RMSE values ranging from 0.27 to 0.43 mm d−1, which demonstrates the adequacy of the latter approach for parameterizing water balance models and for irrigation scheduling decision making.

Production efficiency of ‘BRS Pujante’ cowpea using single and dual Kc for irrigation

ABSTRACT The objective of this study was to evaluate the production efficiency of cowpea (Vigna unguiculata L. Walp.), ‘BRS Pujante’, from the weather-based irrigation scheduling using single and dual Kc. The study was carried out in randomized block design with two treatments (single and dual crop coefficient) and 12 replicates. Chlorophyll a, chlorophyll b and total chlorophyll contents, soil matric tension, number of pods per plant, number of grains per pod, average 100 grains weight, total grain yield and water use efficiency were evaluated. Scheduling with single and dual Kc did not interfere with chlorophyll pigments. Single Kc promoted tension values closer to zero during cowpea development stages III and IV. Significant difference was observed only for the average 100 grains weight, with higher value for dual Kc. The irrigation depth with the single Kc was 360.8 mm and there was a 9.3% water saving when compared to dual Kc, without significantly affecting cowpea yield. It is possible to use irrigation scheduling with single Kc for the stages I and II and dual Kc for the stages III and IV of cowpea, ‘BRS Pujante’.

Diurnal and Seasonal Mapping of Water Deficit Index and Evapotranspiration by an Unmanned Aerial System: A Case Study for Winter Wheat in Denmark

Precision irrigation is a promising method to mitigate the impacts of drought stress on crop production with the optimal use of water resources. However, the reliable assessment of plant water status has not been adequately demonstrated, and unmanned aerial systems (UAS) offer great potential for spatiotemporal improvements. This study utilized UAS equipped with multispectral and thermal sensors to detect and quantify drought stress in winter wheat (Triticum aestivum L.) using the Water Deficit Index (WDI). Biennial field experiments were conducted on coarse sand soil in Denmark and analyses were performed at both diurnal and seasonal timescales. The WDI was significantly correlated with leaf stomatal conductance (R2 = 0.61–0.73), and the correlation was weaker with leaf water potential (R2 = 0.39–0.56) and topsoil water status (the highest R2 of 0.68). A semi-physical model depicting the relationship between WDI and fraction of transpirable soil water (FTSW) in the root zone was derived with R2 = 0.74. Moreover, WDI estimates were improved using an energy balance model with an iterative scheme to estimate the net radiation and land surface temperature, as well as the dual crop coefficient. The diurnal variation in WDI revealed a pattern of the ratio of actual to potential evapotranspiration, being higher in the morning, decreasing at noon hours and ‘recovering’ in the afternoon. Future work should investigate the temporal upscaling of evapotranspiration, which may be used to develop methods for site-specific irrigation recommendations.