Effects of different pipe burial depths on crop yield, water productivity, and irrigation water productivity: A global meta–analysis

Subsurface drip irrigation (SSDI) is an efficient water-saving irrigation technology based on burying drip irrigation pipes underground to deliver water and nutrients directly to crop roots, thereby minimizing water evaporation and nutrient losses. Nevertheless, the effects of SSDI on the crop yield, water productivity (WP), and irrigation water productivity (IWP) have not been fully evaluated, and thus the optimal pipe burial depth for different crops, remains unclear. To address this issue, we performed a global meta-analysis by utilizing 1155 pairs of observations from 145 studies to quantify the effects of different pipe burial depths (< 15 cm, 15–30 cm, and 30–45 cm) on the crop yield, WP, and IWP. Furthermore, we aimed to identify the key factors related to the effects of SSDI on yield increases and water saving. SSDI had significant advantages compared with surface drip irrigation in terms of the crop yield, WP, and IWP, where the optimal performance was obtained at a depth of 30–45 cm (yield increase of 12.30 %, WP increase of 15.26 %, and IWP increase of 18.65 %). SSDI at a depth of 30–45 cm was more favorable for solanaceous vegetables, whereas a depth of 15–30 cm was more suitable for legume crops. Furthermore, field management factors had crucial effects on yield increases and water conservation. An emitter discharge rate of 2.5–3.5 L h^–1 and spacing of 25–35 cm were more favorable for crop growth. In addition, under SSDI conditions, low fertilizer application rates (150 kg ha^–1 N, 50 kg ha^–1 P, and 100–200 kg ha^–1 K) were generally sufficient to achieve the goal of high yields and water savings across different crops, and the effect was particularly pronounced in alkaline (pH ≥ 8), loose (soil bulk density < 1.45 g cm^–3), and nutrient-poor (soil organic matter < 10 g kg^–1) clay soils at 30–45 cm depth. Moreover, the effects of increasing yields and saving water at different pipe burial depths were dramatically affected by climatic factors, with the optimal effect was achieved with precipitation of 400–600 mm and temperatures of 10–15°C at 30–45 cm depth. Our results confirm that SSDI has significant potential for increasing yields and saving water, as well as providing a scientific basis for determining the optimal pipe burial depth for different crop types.