Assessing environmental impacts of agricultural water table management: A global meta-analysis

Global climate change intensifies the need for adaptive water management in agriculture, particularly as extreme rainfall and drought events jeopardize yield stability and water quality. This study provides a comprehensive global meta-analysis comparing controlled drainage (CD) and controlled drainage with subirrigation (CDSI) against free drainage (FD), based on 84 publications encompassing 52 sites (1788 paired site-years). Overall, both CD and CDSI significantly reduced subsurface drainage discharge and nutrient losses (NO₃⁻, TN, DRP, TP) compared to FD, while improving crop yields. CD had no significant effects on surface runoff or surface NO₃⁻ loss, and CDSI increased both. Neither practice significantly influenced subsurface NH₄⁺, PP, or DP losses, and CDSI showed only limited evidence for CH₄ reduction, with no consistent effects on other greenhouse gases. Significant differences between CD and CDSI were observed only for subsurface NO₃⁻, TN, and TP losses, with CDSI achieving greater reductions in NO₃⁻ (CD: 59.4 %, CDSI: 72.5 %) and TN (CD: 16.3 %, CDSI: 55.9 %), whereas CD was more effective in reducing TP (CD: 56.8 %, CDSI: 28.6 %). Moderator effects revealed notable patterns. Crop types influenced subsurface drainage discharge under both CD and CDSI, with small grain crops producing the largest reductions (CD: 68.1 %, CDSI: 83.6 %). Under CDSI, drainage responses were further shaped by N input rate, with higher inputs associated with greater reductions (77.3 %). For CD, subsurface NO₃⁻ loss was strongly affected by precipitation and soil texture, with wetter years (79.2 %) and medium-textured soils (63.6 %) offering the greatest reduction potential. This study presents the first comparative assessment of the environmental impacts of CD and CDSI, highlighting their context-dependent nature and providing insights to inform more informed drainage management decisions. • First comparative assessment of impacts and moderators for controlled drainage and controlled drainage with subirrigation. • Both practices reduced subsurface drainage and nutrient losses relative to free drainage, while also improving crop yields. • Controlled drainage with subirrigation cut nitrate and total nitrogen most; controlled drainage cut total phosphorus most. • Crop types affected subsurface drainage under both ; nitrogen input affected drainage under controlled drainage with subirrigation. • Precipitation and soil texture shaped subsurface nitrate loss under controlled drainage.