(7) Risk and Resilient Decision-Making

Rising aquifer levels due to over-irrigation are contributing to soil-salinization across the Rufiji River Basin [98] and other agricultural areas [99,100]. Moreover, groundwater is increasingly polluted as a result of human activities, notably by nitrates from poor sanitation and fertilizer use [101–103]. Data on hydrogeology is extremely limited [104], ye<sup>t</sup> it is understood that abstractions for domestic uses are growing, despite the severe health risks resulting from polluted groundwater.

One of the most critical challenges faced by the Great Ruaha catchment is the need to adapt to a variable climate, population growth and institutional changes. This means that water reforms should be framed as part of broader-reaching policies such as the education of local communities about watershed protection [87] and the cultivation of drought and salt-resistant crop varieties.

### *3.3. Colorado Basin, Mexico and USA*

The Colorado River (637,137 km2) is an international river shared by two federal countries: Mexico and the USA. It straddles seven states in the USA and two states in Mexico, supporting 2.23 million hectares of irrigated agriculture and 40 million people, including several Tribal Nations [105]. The

basin's multi-purpose reservoir system has the capacity to store approximately four years of annual average runoff (approximately 18.5 BCM), provides 4.2 GW of hydropower capacity, and supports a range of recreational uses, including rafting and boating. The upstream development of water resources has led to the decline of a once-vast delta ecosystem, which is now the focus of bi-national restoration efforts by the US and Mexico to secure water for base flows and pulse flows [18,106].

(1) Well-Defined Reform Objectives

Water allocation in the Colorado River Basin is governed by a complex mix of more than 100 laws, court decisions, operational guidelines, and technical rules known as the "Law of the River". The 1922 Colorado River Compact and the *1928 Boulder Canyon Project Act* established a fixed water allocation for downstream states within the US. This legal framework for interstate apportionment was confirmed in the Supreme Court decision on Arizona v. California in 1963; it requires "upper division" states (Wyoming, Colorado, Utah, and New Mexico) to deliver 92.5 BCM to the "lower division" states (Arizona, California, and Nevada) over a rolling 10-year period. It formally allocated an equivalent volume to the upper division states. Downstream delivery requirements from the upper division to lower division states are assessed on a rolling 10-year accounting period. In practice, the fixed allocation leaves the upper division states with residual flows and, hence, disproportionate exposure to hydro-climatic risks. Both divisions are responsible for Mexico's 1.85 BCM annual allocation secured under a 1944 international treaty.

The overarching reform responds to unsustainable water extractions, which are described as a "structural deficit" in which water use exceeds long-term renewable supplies. In 1999, long-term supply and demand intersected for the first time, coinciding with the beginning of an unprecedented 20-year sequence of dry years and increasing evidence from tree-rings and climate models of the potential for severe sustained drought and drying [107]. Despite over-allocation and a history of disputes, these pressures have prompted a reform period marked by institutional innovations. The states and other stakeholders in the basin have undertaken inter-related investments in institutions, infrastructure, and information to respond to the consequences of climate variability and change, starting with the development of interim guidelines for sharing surplus water among the states in 2001, and six years later, for sharing shortage.
