Study of Spatio-temporal Variability of Water Balance in Semi-arid Region of Southern-India: A case of Cascading Tanks in Madurai
Aman Srivastava and Prof. Pennan Chinnasamy
Most parts of Southern-India is climatically classified from dry sub-humid to semi-arid conditions, especially the states like Tamil Nadu, Andhra Pradesh, and Telangana. These regions have unfortunate historical evidence of witnessing extreme events such as droughts, floods, and cyclones. The reasons have been attributable to the short spell of monsoon season followed by a long spell of dry seasons leading to surplus runoff during rainfall causing limited storage availability in dry periods. This necessitated the creation of surface water storage structures intending to facilitate the water needs during non-monsoon seasons. One such structure is ‘Tank Cascade Systems’ (TCSs) built and maintained for more than 2000 years across dry rural regions of Southern-India. In recent decades, the abrupt changes in the land use-land cover (LULC) pattern and rising population with increased living standards are putting additional stress on water bodies such as TCSs, canals, and groundwater resources for water supply. Simultaneously, the encroachment issues not only declining the command area under tank irrigation but also degrading TCSs as a whole. As a consequence, unmet water demand to meet domestic and irrigation requirements has increased since the last four decades. In modern times, these TCS, though less prevalent, have the potential to face the challenges of the climate change vulnerabilities and anthropogenic stressors such as rapid urbanization and demographic explosion. This calls for the urgency to conduct evidence-based field research over TCSs so as to justify their significance in influencing water balance components thereby micro-climatic conditions.
The present study, for the first time, focuses to document the impacts and challenges faced by TCSs through a real-life case study in Southern-India and provides appropriate sustainable solutions to the degrading TCSs by quantifying the impacts of the modern anthropogenic stressors on the primary functionalities of TCSs. In this context, an extensive field investigation was carried out in the Vandiyur tank cascade system (VTCS) in Madurai city of Tamil Nadu. The study site consisted of six interconnected catchments (C1 to C6) having three developmental settings viz., rural (C1 and C2), peri-urban (C3 and C4) and urban (C5 and C6), each having at least one tank (T) linked with both upstream and downstream tanks through surplus streams. In total, there were eight tanks (T1 to T8) in one of the chains in VTCS with about 22 wells in the vicinity of the VTCS spread area. Given the data collection methods, the study first estimated the seasonal and annual volume of rainfall (P), evaporation (E), and evapotranspiration (ET) based on the magnitudes and rates obtained by analyzing their hydrologic data series. The SCS-CN (Soil Conservation Services Curve Number) method for Indian conditions was used to estimate the catchment runoff (R) for AMC-II (Antecedent Moisture Conditions Type-II characterized by average conditions for soil moisture), individually for rural, peri-urban, and urban landscape for diverse LULC classifications. Reconnaissance survey followed by a detailed field survey and experimentations was conducted across catchments from December 2018 to January 2020 for estimating the storage potential of each tank (S). Based on the magnitudes of P, E, ET, R, and S, the unknown groundwater (G) flow was estimated using the water budget equation. In addition, the field investigation was compounded by studying agricultural practices and measuring rates of municipal water supply in the residential zones, depth to groundwater extraction level in wells, sedimentation load in each tank, and fluctuations in tank water levels across VTCS. The actual volume of each hydrological component was estimated across four seasons viz., winter (January and February), summer (March to May), South-West (SW) monsoon (June to September), and North-East (N-E) monsoon (October to December) in the units of a thousand cubic meters (TCM).
Results indicated a rapid increase in the urban area, up to 300%, in peri-urban and urban catchments. As a consequence, R appeared as the most dominating hydrological parameter representing 40%-60% of P. S was observed highly fluctuating (from 0%-10% during summer to 20%-50% during the N-E monsoon) due to the erratic pattern of P (22% annual deficiency in rainfall) and hence E and G from tanks (Figure 1). Consequently, unmet agricultural water demand was estimated at 90% and 95% during N-E monsoon and S-W monsoon, respectively. Due to inconsistent domestic water supply, unmet domestic water demand was estimated at 73% and 33% during summer and winter, respectively (Figure 2).
The study concluded that the use of a water balance approach in analyzing water scarcity conditions was useful for the catchment development and effective planning of water resource distribution. In general, the application of such hydrological understanding allows the decision-makers to address the unmet water demand given the agricultural practices based on tank irrigation and domestic water supply from the rain-fed dams. Furthermore, it lays the foundation for the development of scalable models for community-led conservation and the development of traditional water resources. In addition, the study provides scopes for appropriate prioritization of sustainable water conservation measures for vulnerable catchments that are capable of bringing best management practices towards revival, rehabilitation, and restoration of traditional water wisdom.
Figure 1: Seasonal average water availability of tanks (T1 to T8) between 2002 and 2020 across Vandiyur Tanks Cascade System (VTCS) in Madurai city of Tamil Nadu
Figure 2: Seasonal domestic water demand and supply (in the units of thousand cubic meters (TCM)) across catchments (C1 to C6) of Vandiyur Tank Cascade System (VTCS) in Madurai city of Tamil Nadu