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Peer-reviewedCC BYInternational Journal of Climatology

Evaluating Reference Evapotranspiration ( ETo ) Calculation Methods and Quantifying Climate‐Driven Change in Irrigation Demand Across Nepal's Terai

  • Umesh Kumar Sujakhu1
  • Rajesh Khatakho2
  • Naresh Suwal3,4
  • Ayush Poudel5
5 affiliations
  1. 1Government of Nepal, Department of Irrigation Kankai Irrigation Management Office Jhapa Koshi Nepal
  2. 2School of Environmental, Civil, Agricultural & Mechanical Engineering University of Georgia Athens Georgia USA
  3. 3Department of Civil and Environmental Engineering Louisiana State University Baton Rouge Louisiana USA
  4. 4Institute for Integrated Hydro Environment Research Kathmandu Bagmati Nepal
  5. 5Department of Biological Systems Engineering Washington State University Pullman Washington USA
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Abstract

Global food security faces growing pressure from population growth and climate change. Improving water productivity requires optimizing irrigation through accurate reference evapotranspiration (ETo) estimates, yet selecting reliable ETo methods remains difficult in data‐scarce Nepal. This study evaluates ETo in the Terai region of Nepal using data from 14 meteorological stations over a historical period (1989–2014) using nine empirical and physically based methods. Coupled Model Intercomparison Project Phase 6 (CMIP6) Global Climate Models (GCMs) data for temperature and precipitation under Shared Socioeconomic Pathways (SSP) SSP245 and SSP585 scenarios were used after downscaling and bias correction to develop a multi‐model ensemble for near (2027–2051), mid (2052–2076), and far‐future (2077–2100) periods. Using these projections, ETo, crop water requirement (CWR), and irrigation water requirements (IWR) were estimated and compared with historical conditions. Among the evaluated models, the Turc (TU) model consistently outperformed the other models, confirming its robustness for ETo estimation. Climate projections indicate increased winter precipitation, particularly under the SSP585 far‐future scenario, alongside rising maximum and minimum temperatures across all scenarios, with stronger warming during winter than the monsoon season. The highest ETo occurred from April to June, and overall ETo is projected to increase under both scenarios, with a pronounced rise (40.7%) in January under SSP585, while slight monsoon‐season declines (0.2%–2.3%) are projected under SSP245. CWR peaks during July–September, with the highest values observed in July. Notable increases in CWR are projected in July under SSP585 mid‐ and far‐future scenarios and during winter across all scenarios. IWR exhibits strong seasonality, peaking in the monsoon months, with most stations showing moderate annual increases (5%–14%), although some stations experience substantial declines (13%–36%). Overall, the results highlight strong spatial heterogeneity in future irrigation demand, emphasizing the need for region‐specific, climate‐resilient water management strategies across the Terai.

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Cite

BibTeXRIS
BibTeX
@article{sujakhu2026evaluating,
  title = {{Evaluating Reference Evapotranspiration ( ETo ) Calculation Methods and Quantifying Climate‐Driven Change in Irrigation Demand Across Nepal's Terai}},
  author = {Umesh Kumar Sujakhu and Rajesh Khatakho and Naresh Suwal and Ayush Poudel},
  journal = {International Journal of Climatology},
  year = {2026},
  month = jun,
  doi = {10.1002/joc.70455},
  url = {https://doi.org/10.1002/joc.70455},
  note = {CC BY},
}
RIS
TY  - JOUR
TI  - Evaluating Reference Evapotranspiration ( ETo ) Calculation Methods and Quantifying Climate‐Driven Change in Irrigation Demand Across Nepal's Terai
AU  - Umesh Kumar Sujakhu
AU  - Rajesh Khatakho
AU  - Naresh Suwal
AU  - Ayush Poudel
PY  - 2026
DA  - 2026/06/02
JO  - International Journal of Climatology
DO  - 10.1002/joc.70455
UR  - https://doi.org/10.1002/joc.70455
AB  - Global food security faces growing pressure from population growth and climate change. Improving water productivity requires optimizing irrigation through accurate reference evapotranspiration (ETo) estimates, yet selecting reliable ETo methods remains difficult in data‐scarce Nepal. This study evaluates ETo in the Terai region of Nepal using data from 14 meteorological stations over a historical period (1989–2014) using nine empirical and physically based methods. Coupled Model Intercomparison Project Phase 6 (CMIP6) Global Climate Models (GCMs) data for temperature and precipitation under Shared Socioeconomic Pathways (SSP) SSP245 and SSP585 scenarios were used after downscaling and bias correction to develop a multi‐model ensemble for near (2027–2051), mid (2052–2076), and far‐future (2077–2100) periods. Using these projections, ETo, crop water requirement (CWR), and irrigation water requirements (IWR) were estimated and compared with historical conditions. Among the evaluated models, the Turc (TU) model consistently outperformed the other models, confirming its robustness for ETo estimation. Climate projections indicate increased winter precipitation, particularly under the SSP585 far‐future scenario, alongside rising maximum and minimum temperatures across all scenarios, with stronger warming during winter than the monsoon season. The highest ETo occurred from April to June, and overall ETo is projected to increase under both scenarios, with a pronounced rise (40.7%) in January under SSP585, while slight monsoon‐season declines (0.2%–2.3%) are projected under SSP245. CWR peaks during July–September, with the highest values observed in July. Notable increases in CWR are projected in July under SSP585 mid‐ and far‐future scenarios and during winter across all scenarios. IWR exhibits strong seasonality, peaking in the monsoon months, with most stations showing moderate annual increases (5%–14%), although some stations experience substantial declines (13%–36%). Overall, the results highlight strong spatial heterogeneity in future irrigation demand, emphasizing the need for region‐specific, climate‐resilient water management strategies across the Terai.
SN  - 0899-8418
SN  - 1097-0088
ER  -