**1. Introduction**

Conflicts over transboundary river sharing due to increases in demand from growing populations and urbanization to produce food and energy are rapidly growing in many basins [1]. Additionally, the impact of climate change is threatening the supply availability of water resources to a significant degree in many locations [2,3]. This imbalance of demand and supply can lead to water insecurity and drive riparian states to engage in unilateral development rather than follow shared water principles, particularly when no prior agreements exist. Clearly, development plans that benefit a given state and potentially result in reduced water availability to other riparian states may be seen as a threat. For this reason, disputes arise among basin countries either to secure their water quota or maintain the status quo. The current conflict over utilization of the Nile River between Ethiopia and Egypt is a prime illustration of this situation.

To settle such kind of controversies associated with border crossing rivers water utilization, the necessity of properly executing international law principles is unquestionable [4]. Among many legal instruments discussed in the next section, the recently ratified and enforced "the 1997 UN Convention on the Law of Non-navigational Uses of International Watercourses (UNWC)" can also be applied. This law is divided by seven parts that contain 37 articles. Particularly, when Article 5 of Part II vividly states the principle of equitable and reasonable water utilization, Article 6 of the convention details relevant factors that should be taken into account to allocate the water in the absence of agreement between riparian countries [5].

However, as M. Franck [6] stated, although a list of criteria or factors is essential for implementing the principle of equitable and reasonable utilization, the seven factors categorized under Article 6 of the UNWC below lack measurability. These factors are;


The difficulty of measuring these broad factors in terms of quantity or specific units also casts doubts on its applicability and interpretation [7].

Only a few studies have addressed describing these factors with measurable indicators. For example, Beaumont [8] suggested two indicators, namely relative flow contribution and prior appropriation, to apply the principle of equitable and reasonable water sharing on transboundary rivers. Ziad and Bassam [9] also proposed nine indicators for the Jordan River basin to allocate water between Israel, Jordan, Palestine, Syria, and Lebanon. With the addition of water quality and ecological variables, a study by Kampragou, et al. [10] proposed 13 additional indicators for equitable water allocation in the Nestos River basin. Although the indicators from these studies provide insight into describing factors in Article 6 of the UNWC, other studies have indicated a lack of consensus among multidisciplinary experts on proposed indicators. Findings from a study by Fariba, et al. [11] on the Sirwan-Diyala River between Iran and Iraq also lacked showing the adequacy of the indicators, both in terms of incorporating the conflicting interests of watercourse states and the scope of all factors.

From the above studies, one can see that the indicators suggested by individual studies are prone to subjective interpretation and open to criticism. The consideration of factors such as environmental, social, economic, and political interests of states have also been disregarded in some studies like that of Beaumont [8]. Moreover, the applicability of specific indicators also varies between locations, and applying only a few variables may not be sufficient to fully consider all dynamics.

Therefore, for a river basin such as the Nile, where only two downstream countries fully control the water based on the 1929 and 1959 colonial treaties and disagreements escalate from time to time [12–17], implementing the principle of equitable and reasonable utilization by identifying measurable indicators is warranted. If not, the Declaration of Principles (DoP) agreed upon in 2015 by Egypt, Ethiopia, and Sudan to utilize the river based on equitable and reasonable sharing remains unfunctional [18].

For this purpose, this study identified and evaluated appropriate indicators for factors mentioned in the UNWC to allocate the Nile River in an equitable and reasonable manner among watercourse states.

#### **2. Background of Legal Instruments**

Following the industrial revolution, as countries started scrambling to meet their ever-increasing water demand, international institutions were required to develop water laws to govern transboundary rivers [19]. Two disparate doctrines were predominantly cited by riparian countries depending on their position. Upstream countries voiced their "absolute sovereign" right to use a river without any restriction, whereas downstream states claimed their right based on the "absolute territorial integrity" principle. Reconciling these two opposing views was difficult, leading to multiple transboundary water allocation conflicts [20]. In 1956, Dubrovnik developed the "limited sovereignty and territorial integrity" principle, which later became a base for the emergence of the principle of equitable and reasonable utilization. In 1966, the International Law Association (ILA), a scholarly but nonauthoritative institution, adopted this principle into the Helsinki rule. This guiding rule became one of the fundamental legal instruments for managing transboundary rivers. Specifically, Article 4 of the rule outlines 11 factors necessary for implementing the principle of equitable and reasonable utilization of international rivers. Though many treaties and agreements were based on this rule, neither scholars in the ILA nor the document are recognized by all countries, leaving the agreements nonbinding [19]. The same is true for the 2004 "Berlin rule," which was extended after the Helsinki rule by ILA. Although the rule is comprehensive in accounting international environmental law, human rights law and the humanitarian law related to management of all kind of water bodies. Similarly, since countries do not agree to be governed by this rule, it is not effective.

Given the increasing number of transboundary water conflicts, the UN General Assembly assigned legal experts nominated by various countries to carefully evaluate international watercourse laws, which eventually became codified in December 1970. Twenty-seven years later, the "UN Convention on the Law of Non-navigational Uses of International Watercourses (UNWC)" was adopted by the general assembly [21]. Whereas the convention also accepted most of the Helsinki rules, major modifications were made including restricting navigational use, the separation of surface and ground water resources, and condensing the original 11 factors into 7. After the ratification process, the UNWC entered into force operationally in 2014, now serving as a key legal instrument used by the international court of justice (ICJ) to resolve international water disputes [20,22]. The heart of the document is Articles 5–7 detailing the principle of equitable and reasonable utilization.

To highlight the abstract meaning and application of the principle of "equitable and reasonable utilization" mentioned under Article 5 [23], Rieu-Clarke, et al. [5] stated that from both procedural and substantive points of view, the term "equity" in international laws implies fairness and justice. Even when UNWC did not exist in its current position, ICJ was applying this interpretation on many international cases, including river basins such as Danube [24,25]. When the procedural interpretation focuses on the right of sovereign states, the substantive dimension seeks to ensure distributive justice. However, "equity" does not simply imply dividing the resource into equal portions [26]. According to McIntyre [27], "equity" can also be viewed from a natural resource allocation perspective. This includes a matter of ensuring certain levels of fairness between developed and developing countries as well as between current and future generations [28,29]. An example of this is the 1992 Rio Declaration [30]. Among 27 principles, principle-16 of the declaration intended to guide countries future sustainable development based on the "polluter pays principle." It established a modality in which developed countries support the green economy of developing countries to balance the harm caused by excessive emission of pollutant gases. Similarly, the term "reasonable" refers to the rationale behind the process of balancing various conflicting uses by states, including the state's development level and other external forces [5]. Again, reasonability does not necessitate achieving optimum allocation or utilization of advanced technology. Rather, it is a relative consideration of current and future contexts without compromising fairness and justice.

Although this principle is fully integrated into the Nile basin's Cooperative Framework Agreement (CFA) and Declaration of Principles (DoP), the CFA lacks a two-third vote necessary for ratification (rejection by Egypt and Sudan), and the DoP agreement only focuses on Ethiopia, Sudan, and Egypt in relation to GERD filling and operation, excluding the six white Nile parties [31,32]. For this reason, the UN convention based on customary international law and applied by the ICJ was utilized as the legal tool in this study.
