Journal of Irrigation and Drainage Engineering

Information on actual consumptive water use and water productivity of wine grape vineyards grown with high-wire cordon (HWC) and vertical shoot position (VSP) trellis systems is limited, but it is critical to inform the selection of resource-efficient grapevine training systems and irrigation water management practices. This article describes an applied-research study conducted in the Central Valley of Chile to determine the actual evapotranspiration (ETa), actual crop coefficients (Ka), water productivity (WP), and water footprint (WFP) of microirrigated wine grape vineyards operated for commercial production with HWC and VSP trellis systems. ETa and Ka were determined using the residual of the energy balance (REB) method from micrometeorological measurements collected at the study vineyards with a combination of surface renewal and eddy covariance equipment. Field measurements of other biophysical parameters were also collected at each study site to characterize the soil water status in multiple locations across the vineyards at various soil depths using moisture tension–monitoring units, vine water status using midday stem water potential $({\mathrm{\Psi }}_{\mathrm{STEM}})$, and fraction of photosynthetically active radiation (fPAR) intercepted by the vine canopy at different times during the growing season. Finally, the fruit yield was determined from grape samples collected at the HWC and VSP vineyard blocks during commercial harvest operations. The study was conducted over the course of two consecutive growing seasons (2019–2020 and 2020–2021). A reduction of 37% in the available water supply occurred during the 2019–2020 season with respect to the average water supply. Findings from the research study showed that when the water supply was less limited, the HWC trellis had 12% higher seasonal cumulative ETa than the VSP trellis, whereas the HWC system appeared more affected by water stress than the VSP trellis during the water-limited season. The analysis of the data sets also showed that the HWC enabled wine grape growers to achieve significantly higher fruit yield and water productivity, and lower water footprint per unit of wine grapes produced during the water-limited season.

Abstract

A study was conducted to test the validity of four different mathematical models imbedded in CropWat software, which is widely used to estimate effective rainfall. The model values were statistically compared with the actual effective rainfall values measured by a field infiltrometer experiment. The study was conducted in a sugarcane land–use system under the tropical climate of Sevanagala, Sri Lanka, during the 2018 to 2019 growing season. The measured effective rainfall in the Sevanagala sugarcane growing area during the 12-month period was $\mathrm{1,068}\mathrm{mm}$, around 70% of the total rainfall of $\mathrm{1,517}\mathrm{mm}$. It was also found that 9% of the rainfall received at the top of the canopy was lost due to canopy interception. The results demonstrated the applicability of the mathematical models embedded in CropWat software to estimate the effective rainfall for sugarcane land–use systems in Sevanagala. Furthermore, the empirical regression formula embedded in CropWat software appeared to be the best-fit model for the estimation of effective rainfall because it had the highest index of agreement value, with an accuracy greater than 96%.

Practical Applications

The current findings can be used to accurately estimate the effective rainfall, which is the actual contribution of rainfall to be used by the sugarcane crop. The results obtains through this process can facilitate managers to prepare plans to reduce irrigation supply without affecting potential crop yields. Furthermore, findings will be useful to technical staff working in the fields of irrigation and water management. The information will be equally helpful to national-level regulatory agencies to prepare feasibility studies and plan future irrigation projects within the country. Irrigation agronomists will also benefit from the paper content because it will help them to handle farm-level operations and irrigation cost-reduction programs. Furthermore, the contents of this article will also be of interest to students and researchers working on hydrological modeling, crop simulation modeling, drought assessment, and empowering sustainable agriculture programs.

A review of the derivation of the Penman–Monteith equation, with the thermodynamic approach of Monteith, is presented. Evapotranspiration (henceforth referred to simply as evaporation) is described as a process consisting of a pair of formal thermodynamic subprocesses (adiabatic cooling and diabatic heating) that leads to changes in the energy states of the ambient air in ways that are readily quantifiable. The Penman–Monteith equation is derived in two steps. As an initial approximation, first a form of the equation that models evaporation into a quiescent ambient air, from a wet source/sink surface, is developed based on thermodynamic equations of state applied to a suitably defined system. In a subsequent step, resistance parameters are introduced into the basic equations, accounting for the dynamic effects of wind-surface interactions and those of (bulk) canopy system effects on evaporation, leading to the Penman–Monteith equation. Although less compact than the conventional approach, the thermodynamic approach to the derivation of the Penman–Monteith equation exhibits a benefit of revealing key assumptions and concepts that are not explicit in the conventional approach. The initial step of the thermodynamic formulation accentuates the notion that the Penman–Monteith equation is fundamentally a description of the process of vapor and heat transfer between a wet source/sink surface and a stationary ambient air. The subsequent step, on the other hand, emphasizes the fact that wind and crop effects on evaporation are taken into account in an approximate sense. The thermodynamic approach also shows that evaporation is essentially a process driven by energy supply and as such, each term of the Penman–Monteith equation represents a separate heat source for evaporation. Furthermore, the approach reveals some useful mathematical/physical attributes of the key parameters of the Penman–Monteith equation. The derivation here emphasizes basic assumptions and mathematical/physical interpretations of results and fills conceptual gaps left from the original discussion. The resultant equations for latent heat flux, sensible heat flux, and the final air temperature represent a coupled set. Thus, numerical solutions to this system of equations are presented and evaluated in a companion manuscript.

A review of the derivation of the Penman–Monteith equation with the thermodynamic approach of Monteith is presented in a companion manuscript. The resultant set of equations (expressed in terms of latent heat flux, ${\mathcal{l}}_f$, sensible heat flux, $q_f$, final air temperature, $T_a$, and the slope parameter related to the saturation vapor pressure curve, $\mathrm{\Delta }$) represents a coupled system. Thus, a pair of alternative numerical solutions, with different levels of complexity, were developed and evaluated in the study reported here. Results showed that the alternative models (labeled as model 1 and 2) produced outputs that are essentially identical and also in close agreement with a reference solution. Intercomparison of the alternative models based on the criteria of numerical efficiency and robustness suggests that each model represents a comparable alternative to the other to estimate evaporation. However, owing to its simplicity, model 1 was selected for further consideration. A comparison of the outputs of model 1 with those of the conventional model (i.e., the approach widely used to evaluate the Penman–Monteith set of equations), based on data sets covering a range of evaporation conditions, showed that the difference in the approaches implemented in the two models has a significant effect on estimates of $q_f$, a limited effect on ${\mathcal{l}}_f$, and a negligible effect on $T_a$. Notably, the results also showed that the mean absolute residual for latent heat flux, ${\mathcal{l}}_f$ (i.e., the mean of the absolute residuals between estimates obtained with model 1 and the conventional model) is relatively small (only about 8.2%), suggesting that differences between ${\mathcal{l}}_f$ estimates computed with model 1 and the conventional model should generally be within the margin of error of the conventional model.

Waters from Rivers Obinna and Adada, located in Enugu, Nigeria, were assessed for their potential for irrigational purposes. Forty-eight water samples were collected from four sampling points during the early dry season and late dry seasons (when these river waters are used for irrigation) from both rivers. Physicochemical parameters were analyzed using standard methods, and seven irrigation water quality indices were applied in ascertaining the suitability of the river waters for irrigation. Mean pH results revealed that the river waters were slightly acidic (6.7). All physicochemical parameters analyzed were within the international standard irrigational water limits, indicating the suitability of the river waters for irrigation. The cations and anions analyzed in the river water were observed to follow the trend ${\mathrm{Ca}}^{2+}{>\mathrm{Mg}}^{2+}{>\mathrm{K}}^{+}{>\mathrm{Na}}^{+}$ and ${\mathrm{Cl}}^{-}{>\mathrm{SO}}_4^{2-}{>\mathrm{NO}}_3^{-}>{\mathrm{PO}}_4^{3-}{>\mathrm{HCO}}_3^{-}{>\mathrm{CO}}_3^{-}$, respectively. Results showed calcium and chloride ions as the most dominant cation and anions, respectively. Piper plots of the anions and cations showed that alkaline earth metals surpassed alkali metal, whereas the weak acids also exceeded the strong acids, with the exception of one sample, which showed dominance of strong acids over weak acids. The observed prevalence of the ${\mathrm{Ca}}^{2+}{-\mathrm{HCO}}_{3-}$ facies was attributed to the large presence of carbonate rocks such as dolomite and calcite, a good indicator of the occurrence of freshwater in both rivers. Positive correlations among the physicochemical parameters was reported, indicating the likelihood of ion exchange between the river waters and surrounding soils. All irrigational water quality indices revealed the suitability of Adada and Obinna Rivers for irrigation applications.