Western Water Security Act of 2019 This bill addresses water management infrastructure and improvement, groundwater management, and water conservation and environmental restoration in certain western states. Specifically, the bill increases the authorization of appropriations to fund research agreements between the Bureau of Reclamation and eligible entities for water management improvement. The bill expands eligibility for such agreements to include nonprofit conservation organizations. The bill also reauthorizes through FY2024 a program through which Interior is authorized to assist eligible desalination projects. The bill lowers cost-sharing requirements under the program for rural desalination projects. Further, the bill allows certain financial assistance made available under an existing drought relief program to be used to assist state and tribal governments in addressing drought-related impacts to water supplies or any other immediate water-related crises. The bill reauthorizes the program through FY2030. Additionally, the bill increases the maximum federal share of costs for certain infrastructure improvements and activities for groundwater management that provide benefits to consumptive water users (i.e., users of water that is not returned to a stream, river, or water treatment plant) and nonconsumptive ecological or recreational values. The bill also requires Reclamation to carry out a water acquisition program in specified river basins. Further, Interior must analyze the extent to which changes in water supply will affect native biodiversity and must produce strategies for sustaining native biodiversity during periods of drought.