Impact of farm dams on streamflow using an objective low-flow threshold from in-stream artificial barriers

Human activity has significant impacts on the river systems. In-stream infrastructure, such as dams and weirs, creates artificial barriers that impede fish passage during periods of low flow; and off-stream structures, such as agricultural farm dams, intercept and extract water. These structures reduce inflow to rivers and extend the frequency and duration of low flow conditions. While the impacts of water interception by farm dams on long-term streamflow have previously been investigated, the consequences for frequency and duration of low flow periods, are poorly understood.  In this study, we estimated the influence of farms dams on annual streamflow and on low flow requirements that ensure water flows over in-stream weirs of different heights for the Moorabool River catchment located in southeast Australia. Streamflow in the Moorabool River was simulated using the hydrological model Genie Rural a 4 parametres Journalier (GR4J) + CHEAT1 to assess the potential impact of farm dams on low flows, using data between 1980 to 2020. The spatial and temporal distribution of farm dams and their storage capacity was estimated using remote sensing. Low flow requirements were calculated measuring cease-to-flow conditions in three weirs in the river, and a low flow spell analysis was conducted to assess the impact of farm dams. From 1990-2020, the number of farm dam increased by 163% across the catchment. Including farm dams and water extractions in the hydrological model increased its performance as assessed by the NSE and logNSE. Streamflow reduction was estimated to be between 3% to 65%, contingent on the level of water extractions and number of dams in the catchment. The cumulative impact of farm dams resulted in fewer high flow events and low flows of longer duration. Prevalent low flows would potentially affect the ability of water to go over some weirs. This study presents a comprehensive approach to quantify water resources by including farm dams and measuring cease-to-flow conditions over weirs in the hydrological model. Ongoing hydrological assessments provides a holistic estimate of water resources and determining objective minimum flow requirements to adequately overtop small weirs in a catchment.  

Keywords: farm dams, hydrological model, low flows, river barriers.