Effects of deficit irrigation and groundwater depth on root growth of direct seeding rice in a column experiment

Rice is an essential crop in Iran that is grown mostly in areas where depth to
groundwater is low. Root growth and water uptake of rice under shallow
groundwater has not been thoroughly studied. This experiment was conducted to
determine the lowland rice (cv. Ghasrodashti) root distribution above shallow
groundwater in relation to deficit irrigation and groundwater depth in cylindrical
greenhouse lysimeters. The irrigation treatments were continuous flood irrigation
(CFI) and intermittent flood irrigation (4- and 8-day intervals IF-4 and IF-8). The
groundwater depths (GWD) were 0.3, 0.45 and 0.6 m from the soil surface. In
general, 40-60% of root dry weight was observed in the top 10 cm of soil in CFI
treatments and IF-4 with 0.3 m GWD resulted in 20% increase in root dry weight
in 10-20 cm layer compared with 0-10 cm. However, the root dry weight in 0-10
cm at 0.60 m GWD was 58% lower in intermittent irrigation compared to CFI.
There was no significant difference in mean root length density in IF-4 with 0.3 m
GWD compared with CFI. In general, lowland rice showed good ability to develop
its root system in shallow groundwater level conditions in order to extract
water due to lower soil water content in the intermittent flood irrigations.
Simple equations were presented to predict the groundwater contribution to
evapotranspiration based on the root length density and root weight density.
Therefore, in areas with shallow groundwater depth (up to 0.45 m from the soil
surface) and low potential evapotranspiration, application of IF-4 instead of CFI
can be useful management especially where water scarcity is a serious problem.
Keywords: Groundwater; Root yield; Root length density; Intermittent flood
irrigation; Continuous flood irrigation.