Understanding Equivalence of Equipments between Actual System and WaterGEMs model and Establishing Equivalence of Actual Operating condition with WaterGEMS model.

I have designed a Piped Irrigation network as shown in Figure 1. In this Piped irrigation network (PIN), Water is supplied directly from a reservoir to Water pipes. There are three outlets which are discharging water to Ambience. The Top surface of reservoir is open to atmosphere and all the three outlets are discharging water to ambience which is at atmospheric pressure. The Figure is in Vertical plane which implies that pipes P-1, P-2,  P-3  are horizontal pipes and pipes P-4, P-5, P-6 are vertical pipes which has outlets. Diameters and Length of all pipes are indicated in Figure-1.  I Need a minimum discharge of 211 Liters/Sec from each outlet. Reservoir minimum Elevation needs to be known which can ensure to meet Minimum discharge condition. 

To evaluate this, I have created a WaterGEMs Model- PIN. I will set Elevation of reservoir to a random value and check if demand is met or not. I will continue to increase or decrease the elevation level till required discharge is received from the Outlets.  

I need help with following Queries

1. Does Model "PIN" Correctly represents Physical/ Actual Items/Equipments installed in Figure-1?

2. Is Putting Junctions at J-4,J-5,J-6 correct to represent outlet of Dia 300 mm which is releasing water to ambient? if yes, How?  If not which is the proper node which I should use in place of Junctions?Will the selection of Proper node in J-4,5,6 depends on whether we are choosing Demand Dependent analysis or Pressure Dependent analysis? In other words, for modeling an outlet as shown in figure-1, a different type of node is suitable for DDA and a different type of node is suitable if we are using  PDA?

3. How Can I set Node properties (of Suitable node as suggested against Question number-2) at J-4,5,6 to establish the actual condition of Discharge of water from outlet  to Ambience. 

4. Is WaterGEMs Model inherently applies one atmosphere pressure at the TOP to Reservoir surface? If not, How I as a User set the system so that my model represent 1 Atmosphere pressure. 

I would request provide detailed answers to these Queries and provide correct model ".wtg" file if possible. Thanks in Advance to generous OpenFlow forum members. Administrators and Moderators.  


  • Hello Chandan,

    If the water at the demand nodes are discharging to the atmosphere, you can use the discharge to atmosphere element. You would enter a typical flow and typical pressure drop based on the opening, and WaterGEMS will calculate the flow from the element. See this link for details, including defining the typical flow and pressure drop: How does WaterGEMS/WaterCAD treat the discharge to atmosphere element?

    Additional ways of modeling demands that vary with pressure can be found here: Options for modeling outflow that vary with pressure.

    Reservoirs are assumed to be at a constant hydraulic grade. Anything above a reservoir or tank is assumed to be at atmospheric pressure.



  • Sir, there are two articles on how to calculate typical flow and typical pressure for Discharge to Atmosphere element. 

    For a required typical flow of 2.11 cubic meters per second,

    Process-1. If we use method described in "Modeling Reference - Discharge To Atmosphere" under section "Common applications of the D2A acting as an Orifice" undersubsection " 2. Any free discharge point" , We get typical pressure drop of 1.263 meter.

    Process-2. If we use method described in "How does WaterGEMS/WaterCAD treat the discharge to atmosphere element?" under section "Method 1: If there is no restriction/contraction at the pipe outlet, then consider using the minor loss equation", We get typical pressure drop of 0.458 meter. Please let me know which is correct Typical pressure drop value. 

  • Hi Chandan,

    • Discharge to Atmosphere (D2A) model elements are primarily aimed at models that are doing Transient / Water Hammer analysis in Hammer, and they originate from when Hammer was a completely separate piece of software before Hammer and WaterCAD were integrated together onto a common platform.  They "can" be used in Steady State simulation engines such as in WaterCAD, but this is using it in a way it was not originally designed to do, although the development team have done a good job of making them useful in Steady State software 
    • If it was me, I would instead use Junction Emitters or Hydrant Emitters if the model is not being used for Water Hammer analysis.  Emitters were created in EPANet and WaterCAD expressly for the design scenario you are considering, to calculate Pressure Dependent Demands (PDDs)

    Out of the two:  Junctions vs Hydrant Emitters I would choose Hydrants.  They are foundationally the same core modelling element but Hydrants are more flexible. In WaterCAD, "Hydrants" are actually hydraulically Junctions but with a couple of added options:

    • Hydrants can be set to Open or Closed Status.  In the model this switches On/Off whether it is an Emitter.  With the Hydrant "Open" then it will discharge a flow rate equal to Q = Emitter Coefficient x Pressure ^ n  (Where n is usually considered to be 0.5 in conventional hydraulics representing the Fixed Orifice equation) 
    • Optionally Hydrants can also model an extra length of Dummy Pipe connected to its Node called a "Hydrant Lateral".  Rather than calculate and input an Emitter Coefficient, the modeller can instead input a Lateral Pipe Length, Diameter, any the Sum of Bend/Fitting/Valve Minor Loss Coefficients.   WaterCAD will take this information to calculate what is the "effective" Emitter Coefficient for this modelled pipework extension to calculate what will be the Demand from the irrigation head.

    So Hydrant Option 1 is to configure it like this.  I've used 45 L/s/m^0.5 as an approximation, but the engineer needs to calculate this value themselves on Eg. Spreadsheet/Calculator using hydraulic design information

    Hydrant Option 2 is to not use an Emitter Coefficient but to use Hydrant Later Pipe Dimensions that give the same effective Emitter Coefficient.  You will need to make the Emitter Coefficient in this option a really high value like 1,000,000 so that the model does not add any extra head losses on the PDD Flow vs Pressure Head curve.  I would still recommend that the modeller ensure these "Lateral Pipe" values align with a separate due-diligence manual calculation on an Eg. spreadsheet to ensure the modeller has calibrated these to the expected Flow vs Pressure Curve.

  • Thank you sir for such a detailed explanation. However, Can you please let me know how can I calculate Minor loss coefficient. 

  • Hello Chandan,

    If you are using the standard headloss equation, K can be assumed to be 1.0 if the pipe discharges to the atmosphere and is not submerged. See the wiki article on the discharge to atmosphere element for more information.