The advantages and disadvantages of subsurface drip irrigation

　　Drip irrigation has been increasingly used since the 1960s, due to its advantages such as increased productivity and greater water saving. One advantage of subsurface (SDI) and surface drip irrigation (DI) is that it also is a cost-effective method for application of nutrients, pesticides, etc., at frequent intervals throughout the crop-growing season. Although the yield response for many crops indicated that the crop yield under SDI was greater than or equal to that under other irrigation methods, including DI, and required less water in many cases, no clear differences were observed between SDI and DI, since differences in the number of experimental years and sites often yielded differing results. Further, plants under SDI need to distribute their roots into a deep water-supplied area during the early stages of growth, and there are few reports evaluating the effects of this stage. In this study, we measured the evapotranspiration rates, LAI, root activities and shoot dry weights of cabbage grown under DI and SDI (15 cm depth) to clarify the reason for uncertainness in the difference between SDI and DI in each growing stage.  

　　Cabbage evapotranspiration rates were greater under DI and SDI in the early and later stages of growth, respectively (Fig. 1). LAI of cabbage was also greater under DI and SDI in the early and later stages of growth, respectively.

　　The roots penetrated to slightly below 15 cm depth in the early stage of growth 17 days after transplanting (DAT). This means that under SDI, only parts of the roots are likely to reach the water-supplied area. This was one reason why the evapotranspiration rate of cabbage under SDI was lower than that under DI in the early stages of growth before 20 DAT. The root TTC reduction capacity (root activity) of cabbage under SDI increased when its evapotranspiration rate increased in the later stage of growth (Figs. 1 and 2). These results demonstrate that the root activity and evapotranspiration rate of the plant under SDI increased in the later stages of growth, although the evapotranspiration rate was lower and the growth was slower in the early stages of growth than for plants under DI. A nutrient-rich and water-rich environment would be preferable for plants under DI and SDI in the early and later stages of growth, respectively. Thus, plant growth under DI and SDI appears to change according to whether other environmental factors such as temperature and solar radiation are advantageous in the early or later stages of growth. These factors are what created the uncertainness in the difference in shoot dry weight and yield between DI and SDI.

(K. Nakamura and K. Ozawa)

Fig. 1. Evapotranspiration rate of cabbage grown under DI and SDI at each stage of growth. DAT: days after transplanting  Fig. 2. TTC reduction capacity (root activity) of cabbage grown under DI and SDI at 56 DAT (day after transplanting).

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