# 2. Methods

## 2.1.Modeling framework

GLOBIOM is a partial equilibrium model representing land‐use based activities: agriculture, forestry and bioenergy sectors. The model uses a bottom‐up approach to crop, livestock, and forestry production based on detailed biophysical and cost information at the gridcell level, while demand for food and fiber are represented at the level of 30 economic regions. Markets in GLOBIOM consider bilateral trade policies and barriers as well as transportation costs (Mosnier et al., 2014). The bottom‐ up modeling approach of the model considers the biophysical environment of production under multiple management systems for 18 globally produced crop products at a gridcell‐level for four crop management systems: subsistence farming, low input rainfed, high input rainfed, and high input irrigated (Leclère et al., 2014b; von Lampe et al., 2014a). GLOBIOM dynamically models the use of cropland, pasture/grassland, managed and unmanaged forest, and other natural vegetation based on the availability of land and water and the relative profitability of each land sector given land rents and irrigation water costs and nonlinear conversion costs (Havlík et al., 2011b; Robinson et al., 2015, 2014; Schmitz et al., 2014).

GLOBIOM uses the globally gridded simulated crop yields and resource requirements (fertilizer, water for irrigation) from EPIC (Balkovič et al., 2013). The water demanded for irrigation is sourced by surface and groundwater and all irrigated areas are constrained to the water that is physically available after use for domestic and industrial purposes (Palazzo et al., 2018, 2017, Pastor et al., 2016, 2014). Section 1 of the <u>supplementary information</u> provides a more detailed description of the representation of irrigation as a crop production system within the modeling framework.

## 2.2.Scenario description

We developed and implemented several irrigation support scenarios to examine the impacts of investments in irrigation and applied the scenarios in GLBOIOM. In our scenarios, we implement the investment and support for irrigation as different levels of subsidy on the large‐scale capital costs and on‐farm capital costs. Table 2 provides an overview of components that make up the full cost of irrigation based on Rogers et al.’s (1998) conceptual overview of costing water for irrigation. Partial subsidies for the capital costs to build large‐scale dams and water delivery systems are assumed to be publically funded in our investment support scenarios.

Subsidies for capital costs from government agencies or basin authorities are still common in many developed countries (including Canada, Australia, Greece, France, Italy, and Spain) (See Table 1 in Toan, 2016). In many developing countries such as India, Pakistan, and China, all capital costs and part of the operation and maintenance costs are subsidized by state agencies and water user organizations (Toan, 2016). While Turral et al. (2010) broadly define public investment in irrigation as expenditures that create an enabling environment for producing economic output and include “irrigation and drainage development, modernization, institutional reform, improved governance, capacity building, management improvement, creation of farmer organizations, and regulatory oversight, as well as farmers’ investment in joint facilities, wells, and on‐farm water storage and irrigation equipment” (p 553). While many of these activities and expenditures have an economic cost, some are beyond the scope of what can be represented in an economic land use modeling framework and are not considered in our approach.

Additionally, we assume that a portion of the costs from the public sector include training as it is an essential component to effectively use and maintain irrigation systems and is often publicly funded, 7