Subsurface drainage in salt-affected cropland: A tiered meta-analysis linking soil salinity reduction with crop yield responses

Subsurface drainage provides an outlet for excess water and leached salts in salt-affected cropland, but its benefits for crop production are not always straightforward. Many studies report soil salinity and crop yield separately, making it difficult to determine how consistently salinity reduction is accompanied by yield recovery across evidence types. We aimed to quantify salinity and yield responses across drainage evidence types, evaluate their co-occurrence, and interpret heterogeneity and supporting water-productivity evidence for drainage management. The evidence base comprised 1102 effect-size rows from 100 study/site units and 97 source records. The extracted responses were synthesized within predefined evidence tiers using study-level aggregation, clustered bootstrap inference and robust sensitivity analysis. Conservative comparisons with controls indicated a 33.3% reduction in soil salinity (95% clustered-bootstrap confidence interval [CI], 19.1–44.0%) and a 28.3% increase in crop yield (95% CI, 21.6–35.5%). Expanded primary evidence indicated a 57.5% reduction in soil salinity (95% CI, 43.2–69.4%). The corresponding yield increase was 70.4% (95% CI, 37.4–119.8%). After excluding five high-response reclamation cases, the yield estimate decreased to 28.0% (95% CI, 21.9–34.4%). Paired-response analysis indicated a positive association between salinity reduction and yield gain, with a posterior regression slope of 0.34 (95% credible interval: 0.09–0.59). Posterior predictive simulations yielded a median joint-benefit proportion of 0.70 (95% posterior predictive interval: 0.45–0.90). In supportive evidence, source-specific mean water-productivity increases ranged from 5.7% to 121.5% for irrigation-input-based measures and 4.1–25.1% for evapotranspiration-based measures. Overall, subsurface drainage consistently reduced soil salinity in controlled and primary field comparisons, whereas the magnitude of yield gains varied with comparison design and site conditions. Drainage recommendations for salt-affected cropland should therefore consider paired salinity-yield responses rather than salinity reduction alone.