Root Water Uptake and Soil Water Dynamics in a Karst Savanna on the Edwards Plateau, TX

Woody plants are encroaching into a karst savanna on the Edwards Plateau in central Texas, but their impact on hydrology is unclear because of high variability in soil depth and uncertainties about shallow and deep root contributions to water uptake, and water dynamics in rocky soil. The overall objectives of this study are to quantify contributions of shallow and deep roots to water uptake, and to quantify the impact of rock on soil hydraulic properties and water storage. A study was conducted in a karst savanna with ~50% woody cover to monitor spatial and temporal variations in soil moisture and root water uptake with neutron probe and time-domain reflectometry measurements. Bulk density was measured using gamma densitometry. Measurements were made to a depth of 1.6 m in a 25 m 25 m grid (5 m node spacing). The results showed that rock created high spatial variability in water storage. Water storage capacity in the measurement grid ranged from 185 to 401 mm, and coupled with heterogeneous distribution of trees led to high spatial variability in root water uptake. Most of the water uptake came from the upper 1 m of the soil profile, but 10% came from below 1.6 m. This indicated that roots had access to water stored within the bedrock, possibly in soil pockets. Statistical analysis showed that spatial distribution of θ was significantly correlated with rock distribution in the profile. Laboratory evaporation measurements showed that Small volume fractions of rock can increase evaporation from soils by slowing upward movement of water, thereby maintaining capillary connectivity to the surface for a longer period of time. Two simulation models, van Genuchten (VG) and Durner, were compared with the data from evaporation experiments. Results showed that the Durner model was more appropriate than the VG model for describing water retention and hydraulic conductivity of rocky soils

A new scheme to optimize irrigation depth using a numerical model of crop response to irrigation and quantitative weather forecasts

Irrigation management can be improved by utilizing advances in numerical models of water flow in soils that can consider future rainfall by utilizing data from weather forecasts. Toward this end, we developed a numerical scheme to determine optimal irrigation depth on scheduled irrigation days based on a concept of virtual net income as a function of cumulative transpiration over each irrigation interval; this scheme combines a numerical model of crop response to irrigation and quantitative weather forecasts. To evaluate benefits, we compared crop growth and net income of this proposed scheme to those of an automated irrigation method using soil water sensors. Sweet potato (Ipomoea batatas (L.), cv. Kintoki) was grown in 2016 in a sandy field of the Arid Land Research Center, Tottori University, Japan under either a non-optimized automated irrigation or the proposed scheme. Under the proposed scheme, 18% less water was applied, yield increased by 19%, and net income was increased by 25% compared with the results of the automated irrigation system. In addition, soil water content simulated by the proposed scheme was in fair agreement with observed values. Thus, it was shown that the proposed scheme may enhance net income and be a viable alternative for determining irrigation depths

Recovery of a salinized tomato field in a coastal polder after the 2016 Kumamoto Earthquake in Japan

After the 2016 Kumamoto Earthquake (Kumamoto earthquake sequence) of maximum magnitude M7.3 occurred, agricultural fields and crops were significantly damaged by soil liquefaction in lowland polders, which had shallow saline groundwater. A desalinization treatment was required to recover agricultural production in polders in Kumamoto. To understand the soil salinity levels in the root zone, we installed multiple-point measurements using the wireless sensor network system (WSNS) for real-time monitoring of soil moisture (θ ) and bulk soil EC (ECb) in a cherry tomato greenhouse. Pore-water electrical conductivity (ECw) can be estimated as an indicator of the field's salinity level simultaneously. First, we found rapid increases in θ and ECw where soil liquefaction and sand boils occurred. Through soil survey, we confirmed the existence of a large channel beneath one of the liquefaction plots, suggesting considerably high spatial heterogeneity of soil salinity across the greenhouse. Secondly, the high salinity level could not be controlled by drip irrigation at all. Finally, 1-week flooded leaching treatments were carried out; the electrical resistivity tomography (ERT) survey indicated that the salinity level decreased significantly except around the channel plot. The combination of the flooded leaching and natural rainfall was more effective for salt leaching in macropores and channels. Our analysis using the WSNS highlighted three management strategies that may help reduce a polder's salinity level