 This is the outline of a watershed. A watershed is an area of land that channels water from rain and snow into rivers and creeks. Watersheds are useful ways of thinking about surface water management, because all water that falls within a watershed eventually ends up flowing out at one single point on a river. Everyone in the world lives within a watershed. If we know how much water flows into a watershed and how much water is needed for all living things that live within the watershed, then we can understand how to manage our watershed to supply enough water to meet everyone's needs. The way we use and move water can change the way water flows within a watershed. For example, we can keep water from flowing out of a watershed in the wet season by building reservoirs. We can then use that water in the dry season for things like growing rice. However, these changes can have negative impacts. Reservoirs can prevent water from flowing downstream, destroying aquatic habitat. These relationships can be complex and difficult to predict, so we can build computer models of watersheds to help us. Models are a simplified representation of the relationships we see in the real world. Computer models help us to understand how much water is available now and into the future. They also help us understand how our actions might impact the watershed and other things living within it. When we test out an action in a model to see how it will impact other parts of the watershed, we call it a scenario. This is the Stunctionette watershed. These are the main rivers within the watershed. We developed a computer model of this watershed using a program called WEAP. In WEAP, you use different colored objects to create a simple representation of the watershed. Land area is represented by green dots, which are called catchments. You could have one catchment for the entire watershed, or divide the area into many catchments, which is what we did for the Stunctionette watershed model. You tell the model how much it rains on the catchments based on historical measurements from weather stations. We call this input data. The catchments then turn the water into runoff, which flows into the rivers, so we can see how much water is in the rivers in the model. All water that rains within this watershed flows out of the Stunctionette river at the very bottom of the watershed, into the Tonle-Sap River. Let's explore the rest of the watershed and its representation in WEAP. This is the lower part of the watershed, which is the area that floods every year, and the area where much of the rice is grown. In the model, rice paddy fields are included within the catchments as a type of land cover. Let's continue moving upstream in the watershed. As we continue upstream, we move across the portion of the watershed that is regularly flooded and into the areas where many of the villages are. Villages are represented by red dots, called demand nodes. In WEAP, they can take water from the stream, groundwater, or other sources. In the Stunctionette model, most villages take water from groundwater. Groundwater sources are represented by green squares. Let's continue moving upstream over the section of the watershed where there are many fields and villages. Eventually, we reach the Stunctionette reservoir. In WEAP, reservoirs are represented by green triangles. Reservoirs hold water in the model so that it can be distributed by canals, which are represented by orange lines. In the model, some catchments contain rice paddy fields that receive irrigation water from the canals. These catchments are connected to the orange lines with green ones. Through these lines, the area with irrigated paddy fields in the catchments can receive water from the Stunctionette reservoir. By including paddy fields in the catchments, we can estimate how much water is needed to grow certain varieties of rice during the wet and dry season. The model can help us understand if we have enough water for everyone to grow rice twice per year, for example. Now let's go all the way to the upper part of the watershed. This area has many more hills and forests. Like paddy fields, forests are included as a land cover type within the catchments. The land cover of the catchments can change over time and can start out as forest and end up as rubber, cassava, or something else. Once our model is constructed, we make sure all of the main features of the watershed are well represented. When the model is complete, we run it. This is called a simulation. During a simulation, the model recreates reality at certain increments of time, called a time step. The Stunctionette model runs at a weekly time step, which means the model calculates rainfall, streamflow, reservoir volume, irrigation, and other things each week. When we string together many weekly model results, we call this a time series. First, we use the model to simulate the past. We compare what the model simulated with what was observed. This process is called calibration. An important metric used for calibration is streamflow. When we know the model is working well, we can use it to simulate the future and see what the future might look like in our watershed. Let's recap what we've learned. It is useful to think about water management at the scale of the watershed in order to make decisions. Our actions can impact the watershed in many ways, and these relationships are complex. Computer models help us to better understand the impacts of those actions so that we can make good decisions. Thanks for watching!