Assessment of lateral flow and base flow for effective interventions in water conservation

Abstract

Subsurface movement of water has a vital role in the availability of water of an area, such that vertical downward movement of water from root zone region will recharge the groundwater and the lateral movement from the soil moisture or groundwater reservoir will tend to diminish it. The vertical downward movement of infiltrated water gets partially blocked and the accumulated water will induces interflow/lateral flow from a point of higher hydraulic gradient towards the lower hydraulic gradient. The midlands in the State of Kerala are predominantly covered with laterite soil underlain with hard laterite and crystalline rocks. Even though the state of Kerala receives an average annual rainfall of 3000 mm, it experiences severe dry spell during post-monsoon season as groundwater storage is not adequate to meet various demands. With growing variations in the characteristics of precipitation, increasing population and urbanization the infiltration opportunity time for rainwater has decreased resulting in further worsening of the situation. The pertaining issue requires a comprehensive investigation, learning and adoption of effective interventions for water conservation. The most hidden knowledge in this regard is the lack of quantitative information available on the movement of subsurface water. Hence, the present study was carried out to understand two major hydrological processes viz., lateral flow / interflow and groundwater flow in lateritic soils. The research was conducted at KCAET Tavanur campus, Kerala Agricultural University, India. The interflow determination was done in three sites through five experimental set ups. The selected experimental sites were having varied vegetal cover, slope and soil texture. Different techniques were used for the inducement of lateral flow such as rainfall simulation using micro-sprinklers, application of water in trenches and the natural rainfall. Interflow was monitored at three different soil depth zones of 0-40 cm, 40-80 cm and 80-120 cm on the vertical face of the trench made at the downslope side of the water application site. Further, salt (NaCl) tracer was used to determine lateral flow by analyzing salt breakthrough which was recorded through electrical conductivity variations at the monitoring trench face. Tracer analysis for interflow estimation was done through 2 experimental set ups in site 2 and site 3. Base flow monitoring was done through salt (KCl) tracer through five observation wells constructed for this purpose. Instantaneous injection of KCl tracer was done in the injection well and salt breakthrough analysis was carried out in nearby monitoring wells. The study also included simulation of lateral flow and groundwater flow using HYDRUS-2D software. The study revealed that among various soil physical properties, the lateral flow discharge greatly depends on bulk density of soil and soil texture. Further, it was found that an instantaneous application of water on the soil surface and in trench results in more lateral flow than gentle application rates. It was proved from the experiments that NaCl can be treated as an effective tracer to monitor the subsurface flow though molecular diffusion of the salt due to antecedent moisture content remains as a hindrance to its use during rainy season. The study has established that though the infiltration capacity of the lateritic soil is very high, major portion of the infiltrated water moves laterally without reaching the water table. Thus, to increase the groundwater recharge, it is essential to enhance vertical preferential flow through deep rooted vegetation or by deep trenches and pits. Adopting subsurface mechanical barriers which can intercept and divert the interflow to downward direction is also appears to be effective. Further, simulation of lateral flow and base flow using HYDRUS-2D software predicted the lateral flow discharge with a correlation coefficient of 96.4 %. The prediction accuracy of the model for base flow was 73%.