Dangare et al. (2025) Estimating transpiration dynamics of a low-density litchi orchard using crop coefficients derived from a variable leaf conductance model, canopy cover, and tree height in Northeastern South Africa

Identification

• Journal: Irrigation Science
• Year: 2025
• Date: 2025-11-28
• Authors: Prince Dangare, Paul Cronjé, Zama Eric Mashimbye, Tendai Sawunyama, J. Masanganise, Zanele Ntshidi, George P. Nel, Sebinasi Dzikiti
• DOI: 10.1007/s00271-025-01047-4

Research Groups

• Department of Horticultural Science, Stellenbosch University, South Africa
• Citrus Research International, South Africa
• Department of Geography and Environmental Studies, Stellenbosch University, South Africa
• Inkomati – Usuthu Catchment Management Agency, South Africa
• Arid Lands Node, South African Environmental Observation Network (SAEON), South Africa
• Department of Electronics and Telecommunications, University of Zimbabwe, Zimbabwe
• Department of Physical and Earth Sciences, Sol Plaatje University, South Africa
• Department of Physics and Engineering, Bindura University of Science Education, Zimbabwe

Short Summary

This study improved the estimation of litchi orchard transpiration in semi-arid South Africa by modifying the Allen and Pereira (A&P) crop coefficient approach to incorporate a variable leaf resistance model and a litchi-specific typical leaf resistance, achieving significantly higher accuracy compared to the original fixed-value method.

Objective

• To quantify the transpiration and its drivers for a mature litchi orchard growing in a semi-arid environment, addressing an information gap.
• To improve the Allen and Pereira (A&P) method for calculating basal crop coefficients ((\:{\text{K}}_{\text{c}\text{b}})) by implementing a variable rather than a fixed leaf resistance term in the stomatal sensitivity function.

Study Configuration

• Spatial Scale: A 13.1 hectare low-density litchi (Mauritius cultivar) orchard at Riverside farm, Mpumalanga province, Northeastern South Africa (25.447924°S; 31.5547226°E; 144 m above sea level). The orchard had a density of approximately 70 trees per hectare, with an average tree height of 6.3 m.
• Temporal Scale: Two growing seasons, from October 2021 to October 2023.

Methodology and Data

• Models used:
  • Allen and Pereira (A&P) approach for basal crop coefficient ((\:{\text{K}}{\text{c}\text{b}})) calculation, with proposed modifications.
  • Jarvis (1976) leaf resistance model for variable stomatal conductance ((\:{\text{g}}{\text{S}\text{T}})).
  • FAO Penman-Monteith equation for reference evapotranspiration ((\:{\text{E}\text{T}}_{\text{o}})).
  • Heat Ratio Method (HRM) for sap flow measurement.
• Data sources:
  • Microclimate: Automatic weather station (pyranometer, digital probe for air temperature/humidity, sonic anemometer, tipping bucket rain gauge) measuring solar radiation, air temperature, relative humidity, wind speed/direction, and rainfall at hourly intervals.
  • Transpiration: Heat Ratio Method (HRM) sap flow sensors (T-type thermocouples) installed on four representative trees/branches, measured hourly.
  • Irrigation: Automated irrigation scheduled by capacitance probes measuring volumetric soil water content; electronic water flow meter recording hourly total irrigation volumes.
  • Soil Moisture: Soil moisture probes installed at various depths (30, 60, 100, 120, 150 cm) within and between tree rows, recording hourly values. Soil physical properties (field capacity, permanent wilting point) determined by laboratory analysis.
  • Growth/Canopy: Leaf Area Index (LAI) measured every 3 months using a canopy analyzer; missing values interpolated using cubic spline. Tree height measured with a pole.
  • Leaf Gas Exchange: Leaf stomatal conductance ((\:{\text{g}}_{\text{S}\text{T}})) measured hourly on selected clear days using an infrared gas analyzer (IRGA).

Main Results

• A litchi-specific typical leaf resistance ((\:{\text{r}}_{\text{t}\text{y}\text{p}})) of 55 s/m was derived, replacing the standard A&P value of 100 s/m (derived for annual crops).
• The improved A&P method, incorporating a variable leaf resistance (modeled by the Jarvis model) and the litchi-specific (\:{\text{r}}_{\text{t}\text{y}\text{p}}) of 55 s/m, significantly improved transpiration estimation.
  • Comparison of calculated and measured transpiration rates yielded: Coefficient of determination ((\:{\text{R}}^{2})) = 0.82, Normalized Root Mean Square Error (NRMSE) = 0.12, Normalized Mean Absolute Error (NMAE) = 0.10, and Nash-Sutcliffe Efficiency (NSE) = 0.64.
• The original A&P method (fixed leaf resistance, (\:{\text{r}}_{\text{t}\text{y}\text{p}}) = 100 s/m) showed poor performance (NSE = 0.05) and significantly overestimated transpiration by 18% over two years.
• The improved model underestimated measured transpiration by 0.24% over two years.
• The Jarvis stomatal conductance model accurately predicted litchi stomatal conductance, with (\:{\text{R}}^{2}) values of 0.74 (winter), 0.72 (spring), and 0.67 (summer) during validation, and acceptable NSE values (0.54, 0.47, 0.48, respectively).
• Average annual litchi orchard transpiration was 302 mm (3020 m³/ha).
• Solar radiation was identified as the strongest driver of litchi transpiration ((\:{\text{R}}^{2}) = 0.91). Transpiration response to VPD was non-linear, increasing up to 2.0 kPa, then decreasing due to stomatal closure.

Contributions

• Provided the first accurate quantitative information on transpiration and its drivers for ultra-low density, multi-stemmed litchi orchards under semi-arid subtropical conditions.
• Developed and validated an improved Allen and Pereira (A&P) crop coefficient approach for tree crops by replacing the fixed leaf resistance with a variable leaf resistance model (Jarvis model) and deriving a crop-specific typical leaf resistance ((\:{\text{r}}_{\text{t}\text{y}\text{p}}) = 55 s/m for litchi).
• Demonstrated that the improved method significantly enhances the accuracy of litchi orchard transpiration estimation, making it suitable for integration into precision irrigation decision support systems without adding complexity.
• Showcased the seasonal variability of stomatal conductance in litchi trees and the importance of season-specific model calibration.

Funding

• Water Research Commission (WRC) - Project WRC C2020/2023 − 00399 ("Developing a decision support system for estimating the water use and efficiency of irrigated crops in the Inkomati-Usuthu Water Management Area (WMA)").
• Inkomati-Usuthu Catchment Management Agency (South Africa).

Citation

@article{Dangare2025Estimating,
  author = {Dangare, Prince and Cronjé, Paul and Mashimbye, Zama Eric and Sawunyama, Tendai and Masanganise, J. and Ntshidi, Zanele and Nel, George P. and Dzikiti, Sebinasi},
  title = {Estimating transpiration dynamics of a low-density litchi orchard using crop coefficients derived from a variable leaf conductance model, canopy cover, and tree height in Northeastern South Africa},
  journal = {Irrigation Science},
  year = {2025},
  doi = {10.1007/s00271-025-01047-4},
  url = {https://doi.org/10.1007/s00271-025-01047-4}
}

Generated by BiblioAssistant using gemini-2.5-flash (Google API)