diff --git a/OpenHPL/Waterway/BendPipe.mo b/OpenHPL/Waterway/BendPipe.mo
index ea3ce82..6086982 100644
--- a/OpenHPL/Waterway/BendPipe.mo
+++ b/OpenHPL/Waterway/BendPipe.mo
@@ -8,15 +8,22 @@ model BendPipe "Bend in pipes"
     Dialog(group = "Manufacturer's design"));
   /* geometrical parameters for fitting */
   parameter SI.Diameter D_i = 3 "Pipe diameter of the inlet (LHS)" annotation (
-    Dialog(group = "Geometry"));
+    Dialog(group = "Geometry", enable=not use_A));
   parameter SI.Diameter D_o = D_i "Pipe diameter of the outlet (RHS)" annotation (
-    Dialog(group = "Geometry"));
+    Dialog(group = "Geometry", enable=not use_A));
+  parameter Boolean use_A = false "Use areas instead of diameters" annotation (
+    Dialog(group = "Geometry"), choices(checkBox=true));
+  parameter SI.Area A_i = 7.07 "Area of the inlet side" annotation (
+    Dialog(group = "Geometry", enable=use_A));
+  parameter SI.Area A_o = A_i "Area of the outlet side" annotation (
+    Dialog(group = "Geometry", enable=use_A));
   SI.Velocity v(start=Modelica.Constants.eps) "Water velocity";
-  SI.Area A = pi*D_i^2/4 "Cross section area";
+  SI.Area A "Cross section area";
   SI.Pressure dp "Pressure drop of fitting";
   SI.MassFlowRate mdot "Mass flow rate";
   extends OpenHPL.Interfaces.TwoContacts;
 equation
+  A = if use_A then (if mdot >= 0 then A_i else A_o) else pi * D_i ^ 2 / 4;
   v = mdot / data.rho / A;
   dp = K_L * 0.5 * data.rho * v^2;
    // Connections
diff --git a/OpenHPL/Waterway/DraftTube.mo b/OpenHPL/Waterway/DraftTube.mo
index 97f0d44..d7544e6 100644
--- a/OpenHPL/Waterway/DraftTube.mo
+++ b/OpenHPL/Waterway/DraftTube.mo
@@ -2,7 +2,7 @@ within OpenHPL.Waterway;
 model DraftTube "Model of a draft tube for reaction turbines"
   outer Data data "Using standard data set";
   extends OpenHPL.Icons.DraftTube;
-  extends Types.FrictionSpec( final D_h = 0.5 * (D_i + D_o));
+  extends Types.FrictionSpec(final D_h = D_);
   import Modelica.Constants.pi;
   parameter Types.DraftTube DraftTubeType = OpenHPL.Types.DraftTube.ConicalDiffuser "Types of draft tube" annotation (
     Dialog(group = "Draft tube types"));
@@ -13,9 +13,15 @@ model DraftTube "Model of a draft tube for reaction turbines"
   parameter SI.Length L = 7.017 "Centerline/slant length (conical diffuser). Inclination is derived from H and L." annotation (
     Dialog(group = "Geometry",enable=DraftTubeType == OpenHPL.Types.DraftTube.ConicalDiffuser));
   parameter SI.Diameter D_i = 4 "Diameter of the inlet side" annotation (
-    Dialog(group = "Geometry"));
+    Dialog(group = "Geometry", enable=not use_A));
   parameter SI.Diameter D_o = 4.978 "Diameter of the outlet side" annotation (
-    Dialog(group = "Geometry"));
+    Dialog(group = "Geometry", enable=not use_A));
+  parameter Boolean use_A = false "Use areas instead of diameters" annotation (
+    Dialog(group = "Geometry"), choices(checkBox=true));
+  parameter SI.Area A_i = 12.57 "Area of the inlet side" annotation (
+    Dialog(group = "Geometry", enable=use_A));
+  parameter SI.Area A_o = 19.46 "Area of the outlet side" annotation (
+    Dialog(group = "Geometry", enable=use_A));
 
   parameter SI.Length L_m = 4 "Length of main section (Moody spreading pipe)" annotation (
     Dialog(group = "Geometry",enable=DraftTubeType == OpenHPL.Types.DraftTube.MoodySpreadingPipe));
@@ -35,12 +41,12 @@ model DraftTube "Model of a draft tube for reaction turbines"
   // parameter Boolean TempUse = data.TempUse "If checked - the water temperature is not constant" annotation (Dialog(group = "Initialization"));
   // parameter SI.Temperature T_0 = data.T_0 "Initial water temperature in the pipe" annotation (Dialog(group = "Initialization", enable = TempUse));
 
-  /* variables */
-  SI.Diameter D_ = 0.5 * (D_i + D_o) "Average diameter";
-  SI.Area A_i = D_i ^ 2 * pi / 4 "Inlet cross-section area of draft tube";
-  SI.Area A_o = D_o ^ 2 * pi / 4 "Outlet cross-section area of draft tube";
-  SI.Area A_ = D_ ^ 2 * pi / 4 "Average cross-section area of conical diffuser";
 
+  protected
+  parameter SI.Diameter D_ = if use_A then sqrt(4 * A_ / pi) else 0.5 * (D_i_ + D_o_) "Average diameter";
+  parameter SI.Area A_ = if use_A then (A_i + A_o) / 2 else D_ ^ 2 * pi / 4 "Average cross-section area of conical diffuser";
+
+  /* variables */
   SI.Mass m "Mass of water inside conical diffuser";
   SI.Mass m_m "Mass of water inside Main section Moody spreading pipes";
   SI.Mass m_b "Mass of water inside Branch section Moody spreading pipes";
@@ -79,6 +85,9 @@ model DraftTube "Model of a draft tube for reaction turbines"
 
  // connectors
   extends OpenHPL.Interfaces.TwoContacts;
+protected
+  parameter SI.Diameter D_i_ = if use_A then sqrt(4 * A_i / pi) else D_i;
+  parameter SI.Diameter D_o_ = if use_A then sqrt(4 * A_o / pi) else D_o;
 initial equation
   if SteadyState then
     der(M) = 0;
@@ -88,7 +97,7 @@ initial equation
     //n.T = p.T;
   end if;
 equation
-  assert(D_i > 0 and D_o > 0, "DraftTube diameters D_i and D_o must be > 0.", AssertionLevel.error);
+  assert(D_i_ > 0 and D_o_ > 0, "DraftTube areas or diameters must be > 0.", AssertionLevel.error);
 
   der(M) = Mdot + F "Momentum balance";
   if DraftTubeType == OpenHPL.Types.DraftTube.ConicalDiffuser then
@@ -98,7 +107,7 @@ equation
     M = m*v;
     m = data.rho*V "Mass of water inside the draft tube";
     m_m=0;m_b=0; // Unimportant for conical diffuser
-    V = pi*H/12*(D_i^2+D_o^2+D_i*D_o) "Volume of water inside the draft tube";
+    V = pi*H/12*(D_i_^2+D_o_^2+D_i_*D_o_) "Volume of water inside the draft tube";
     v = Vdot/A_;
     Vdot_b = 0; // Unimportant for conical diffuser
     v_m=0;v_b=0; // Unimportant for conical diffuser
@@ -111,7 +120,7 @@ equation
     F_p = p_i * A_i - p_o * A_o;
     F_f = Functions.DarcyFriction.Friction(v, D_, L, data.rho, data.mu, p_eps)+1/2*data.rho*v*abs(v)*A_i*phi_d;
     F_fm=0;F_fb=0;
-    phi_d = 0.23*(1-D_i/D_o)^2;
+    phi_d = 0.23*(1-D_i_/D_o_)^2;
     phi_d_o=0; // Unimportant for conical diffuser
     F_g = m*data.g*cos_theta;
 
@@ -131,8 +140,8 @@ equation
     F_p = p_i*A_i-2*p_o*A_o*cos_theta_moody_by_2;
     F_g = m_m*data.g+2*m_b*data.g*cos_theta_moody_by_2;
     F_f = F_fm+2*F_fb*cos_theta_moody_by_2+data.rho*v_m*abs(v_m)*A_i*phi_d;
-    F_fm = Functions.DarcyFriction.Friction(v_m, D_i, L_m, data.rho, data.mu, p_eps);
-    F_fb = Functions.DarcyFriction.Friction(v_b, D_o, L_b, data.rho, data.mu, p_eps);
+    F_fm = Functions.DarcyFriction.Friction(v_m, D_i_, L_m, data.rho, data.mu, p_eps);
+    F_fb = Functions.DarcyFriction.Friction(v_b, D_o_, L_b, data.rho, data.mu, p_eps);
 
      phi_d = 1+(v_b/v_m)^2-2*v_b/v_m*cos_theta_moody-phi_d_o*(v_b/v_m)^2;
 
@@ -159,8 +168,8 @@ equation
     <p>Two draft tube variants are modeled using <em>momentum balance</em>.</p>
 <p><strong>Parameterization:</strong></p>
 <ul>
-<li><strong>Conical diffuser:</strong> use <code>H</code>, <code>L</code>, <code>D_i</code>, and <code>D_o</code>. The inclination is derived internally from the slope ratio <code>H/L</code>.</li>
-<li><strong>Moody spreading pipe:</strong> use <code>L_m</code>, <code>L_b</code>, <code>D_i</code>, <code>D_o</code>, and <code>theta_moody</code>.</li>
+<li><strong>Conical diffuser:</strong> use <code>H</code>, <code>L</code>, and inlet/outlet diameters <code>D_i</code>/<code>D_o</code> or areas <code>A_i</code>/<code>A_o</code>. The inclination is derived internally from the slope ratio <code>H/L</code>.</li>
+<li><strong>Moody spreading pipe:</strong> use <code>L_m</code>, <code>L_b</code>, inlet/outlet diameters <code>D_i</code>/<code>D_o</code> or areas <code>A_i</code>/<code>A_o</code>, and <code>theta_moody</code>.</li>
 </ul>
 <p>They are:</p>
 <ul>
diff --git a/OpenHPL/Waterway/Fitting.mo b/OpenHPL/Waterway/Fitting.mo
index b256690..084039f 100644
--- a/OpenHPL/Waterway/Fitting.mo
+++ b/OpenHPL/Waterway/Fitting.mo
@@ -7,11 +7,21 @@ model Fitting "Different pipes fitting"
   parameter Types.Fitting fit_type=OpenHPL.Types.Fitting.Square "Type of pipe fitting";
   /* geometrical parameters for fitting */
   parameter SI.Diameter D_i = 5.8 "Pipe diameter of the inlet (LHS)" annotation (
-    Dialog(group = "Geometry"));
+    Dialog(group = "Geometry", enable=not use_A));
   parameter SI.Diameter D_o = 3.3 "Pipe diameter of the outlet (RHS)" annotation (
-    Dialog(group = "Geometry"));
+    Dialog(group = "Geometry", enable=not use_A));
+  parameter Boolean use_A = false "Use areas instead of diameters" annotation (
+    Dialog(group = "Geometry"), choices(checkBox=true));
+  parameter SI.Area A_i = 26.42 "Area of the inlet side" annotation (
+    Dialog(group = "Geometry", enable=use_A));
+  parameter SI.Area A_o = 8.55 "Area of the outlet side" annotation (
+    Dialog(group = "Geometry", enable=use_A));
+protected
+  parameter SI.Diameter D_i_ = if use_A then sqrt(4 * A_i / pi) else D_i;
+  parameter SI.Diameter D_o_ = if use_A then sqrt(4 * A_o / pi) else D_o;
+public
   parameter Modelica.Units.NonSI.Angle_deg theta=45 "If Tapered fitting: angle of the tapered reduction/expansion" annotation (Dialog(group="Geometry", enable=fit_type == OpenHPL.Types.Fitting.Tapered));
-  parameter SI.Length L(max = 5 * D_o) = 1 "If Thick Orifice: length of the thick orifice, condition L/D_2<=5. If this condition is not satisfied (L is longer) then use Square Reduction followed by Square Expansion" annotation (
+  parameter SI.Length L(max = 5 * D_o_) = 1 "If Thick Orifice: length of the thick orifice, condition L/D_2<=5. If this condition is not satisfied (L is longer) then use Square Reduction followed by Square Expansion" annotation (
     Dialog(group = "Geometry", enable=fit_type == OpenHPL.Types.Fitting.ThickOrifice));
   /* variables */
   SI.Velocity v(start=Modelica.Constants.eps) "Water velocity";
@@ -26,28 +36,28 @@ equation
   if v>=0 then
     phi =Functions.Fitting.FittingPhi(
     v,
-    D_i,
-    D_o,
+    D_i_,
+    D_o_,
     L,
     theta,
     data.rho,
     data.mu,
     data.p_eps,
     fit_type);
-    A = pi * D_i^2 / 4;
+    A = if use_A then A_i else pi * D_i^2 / 4;
     dp = phi * 0.5 * data.rho * v^2;
   else
     phi =Functions.Fitting.FittingPhi(
     v,
-    D_o,
-    D_i,
+    D_o_,
+    D_i_,
     L,
     theta,
     data.rho,
     data.mu,
     data.p_eps,
     fit_type);
-    A = pi * D_o^2 / 4;
+    A = if use_A then A_o else pi * D_o^2 / 4;
     dp = - phi * 0.5 * data.rho * v^2;
   end if;
   o.p = i.p - dp "Pressure of the output connector";
@@ -56,7 +66,7 @@ equation
   o.elevation.z = i.elevation.z "Elevation propagation: no height change across fitting";
   annotation (preferredView="info",
     Documentation(info="<html>
-    <p>Various possibilities of the fittings for the pipes with different diameters
+    <p>Various possibilities of the fittings for the pipes with different diameters or cross-sectional areas
     and also the orifices in the pipe.
     Here, the pressure drop due to these constrictions is defined.</p>
 <table cellspacing=\"0\" cellpadding=\"2\" border=\"0\" width=\"50%\"><tr>
diff --git a/OpenHPL/Waterway/Penstock.mo b/OpenHPL/Waterway/Penstock.mo
index 828c326..bbdb8ad 100644
--- a/OpenHPL/Waterway/Penstock.mo
+++ b/OpenHPL/Waterway/Penstock.mo
@@ -10,14 +10,22 @@ model Penstock "Model of the penstock with elastic walls and compressible water.
   parameter SI.Length L = 600 "length of the pipe, m" annotation (
     Dialog(group = "Geometry"));
   parameter SI.Diameter D_i = 3.3 "Diametr from the input side of the pipe" annotation (
-    Dialog(group = "Geometry"));
+    Dialog(group = "Geometry", enable=not use_A));
   parameter SI.Diameter D_o = D_i "Diametr from the output side of the pipe" annotation (
-    Dialog(group = "Geometry"));
+    Dialog(group = "Geometry", enable=not use_A));
+  parameter Boolean use_A = false "Use areas instead of diameters" annotation (
+    Dialog(group = "Geometry"), choices(checkBox=true));
+  parameter SI.Area A_i = 8.55 "Area of the inlet side" annotation (
+    Dialog(group = "Geometry", enable=use_A));
+  parameter SI.Area A_o = A_i "Area of the outlet side" annotation (
+    Dialog(group = "Geometry", enable=use_A));
+  parameter SI.Diameter D_i_ = if use_A then sqrt(4 * A_i / pi) else D_i;
+  parameter SI.Diameter D_o_ = if use_A then sqrt(4 * A_o / pi) else D_o;
   parameter SI.VolumeFlowRate Vdot_0 = 20 "Initial volume flow rate" annotation (
     Dialog(group = "Initialization"));
   parameter Integer N = 20 "Number of segments" annotation (
     Dialog(group = "Discretization"));
-  SI.Diameter dD = 0.5 * (D_i + D_o), D[N] = linspace(D_i + dD / 2, D_o - dD / 2, N), D_[N + 1] = linspace(D_i, D_o, N + 1);
+  SI.Diameter dD = 0.5 * (D_i_ + D_o_), D[N] = linspace(D_i_ + dD / 2, D_o_ - dD / 2, N), D_[N + 1] = linspace(D_i_, D_o_, N + 1);
   SI.Area A[N] = D .^ 2 * pi / 4, A_[N + 1] = D_ .^ 2 * pi / 4, A_m[N - 2], A_m_end, A_m_first;
   SI.Pressure p_i, p_o, p_[N - 1], dp = data.rho * data.g * H / N, p_m[N - 2];
   SI.Length dx = L / N, Per_m[N - 2];
diff --git a/OpenHPL/Waterway/PenstockKP.mo b/OpenHPL/Waterway/PenstockKP.mo
index 3d46396..eae77d1 100644
--- a/OpenHPL/Waterway/PenstockKP.mo
+++ b/OpenHPL/Waterway/PenstockKP.mo
@@ -2,7 +2,7 @@ within OpenHPL.Waterway;
 model PenstockKP "Detailed model of the pipe. Could have elastic walls and compressible water. KP scheme"
   outer OpenHPL.Data data "Using standard data set";
   extends OpenHPL.Icons.Pipe( slanted=true);
-  extends Types.FrictionSpec( final D_h = (D_i + D_o) / 2);
+  extends Types.FrictionSpec(final D_h = (D_i_ + D_o_) / 2);
   import Modelica.Constants.pi;
   // geometrical parameters of the pipe
   parameter SI.Height H = 420 "Height difference from the inlet to the outlet of the pipe" annotation (
@@ -10,9 +10,17 @@ model PenstockKP "Detailed model of the pipe. Could have elastic walls and compr
   parameter SI.Length L = 600 "length of the pipe" annotation (
     Dialog(group = "Geometry"));
   parameter SI.Diameter D_i = 3.3 "Diametr from the inlet side of the pipe" annotation (
-    Dialog(group = "Geometry"));
+    Dialog(group = "Geometry", enable=not use_A));
   parameter SI.Diameter D_o = D_i "Diametr from the outlet side of the pipe" annotation (
-    Dialog(group = "Geometry"));
+    Dialog(group = "Geometry", enable=not use_A));
+  parameter Boolean use_A = false "Use areas instead of diameters" annotation (
+    Dialog(group = "Geometry"), choices(checkBox=true));
+  parameter SI.Area A_i = 8.55 "Area of the inlet side" annotation (
+    Dialog(group = "Geometry", enable=use_A));
+  parameter SI.Area A_o = A_i "Area of the outlet side" annotation (
+    Dialog(group = "Geometry", enable=use_A));
+  parameter SI.Diameter D_i_ = if use_A then sqrt(4 * A_i / pi) else D_i;
+  parameter SI.Diameter D_o_ = if use_A then sqrt(4 * A_o / pi) else D_o;
   // condition of steady state
   parameter Boolean SteadyState=data.SteadyState "If true, starts in steady state" annotation (Dialog(group="Initialization"));
   // staedy state values for flow rate in all segments of the pipe
@@ -30,7 +38,7 @@ model PenstockKP "Detailed model of the pipe. Could have elastic walls and compr
     choices(checkBox = true),
     Dialog(group = "Properties"));
   // variables
-  SI.Diameter dD = (D_i - D_o) / N "step in diameter change", D[N] = linspace(D_i + dD / 2, D_o - dD / 2, N) "centered diameter vector in atm. p.", D_[N + 1] = linspace(D_i, D_o, N + 1) "boundary diameter vector in atm. p.";
+  SI.Diameter dD = (D_i_ - D_o_) / N "step in diameter change", D[N] = linspace(D_i_ + dD / 2, D_o_ - dD / 2, N) "centered diameter vector in atm. p.", D_[N + 1] = linspace(D_i_, D_o_, N + 1) "boundary diameter vector in atm. p.";
   SI.Area A_atm[N] = D .* D * pi / 4 "centered cross are vector in atm. p.", A_atm_[N + 1] = D_ .* D_ * pi / 4 "boundary cross are vector in atm. p.", A[N] "centered cross are vector", A_[N, 4] "boundary cross are vector", _A_atm[N, 4] "boundary cross are matrix in atm. p.";
   SI.Pressure p_p[N] "centered pressure", dp = data.rho * data.g * H / N "initial p. step", p_i "Inlet pressure (LHS)", p_o "Outlet Pressure (RHS)", p_[N, 4] "boundary p. matrix";
   SI.Length dx = L / N "length step", dh = H / N "height step";
@@ -167,7 +175,7 @@ c = √(A<sub>p</sub>/(A<sub>p</sub><sup>atm</sup> ρ<sup>atm</sup> β<sup>tot</
 <h5>Parameters</h5>
 <p>User specifies:</p>
 <ul>
-<li>Geometry: length <code>L</code>, height difference <code>H</code>, inlet/outlet diameters <code>D_i</code>/<code>D_o</code>,
+<li>Geometry: length <code>L</code>, height difference <code>H</code>, inlet/outlet diameters <code>D_i</code>/<code>D_o</code> or areas <code>A_i</code>/<code>A_o</code>,
 roughness <code>p_eps</code></li>
 <li>Discretization: number of cells <code>N</code></li>
 <li>Model fidelity: <code>PipeElasticity</code> checkbox to enable/disable elastic walls</li>
diff --git a/OpenHPL/Waterway/Pipe.mo b/OpenHPL/Waterway/Pipe.mo
index 2600893..d9a2d81 100644
--- a/OpenHPL/Waterway/Pipe.mo
+++ b/OpenHPL/Waterway/Pipe.mo
@@ -3,17 +3,24 @@ model Pipe "Model of a pipe"
   outer Data data "Using standard data set";
   extends OpenHPL.Icons.Pipe;
   extends OpenHPL.Interfaces.TwoContacts;
-  extends Types.FrictionSpec( final D_h = (D_i + D_o) / 2);
+  extends Types.FrictionSpec(final D_h = D_);
 
   // Geometrical parameters of the pipe:
   parameter SI.Length H = 0 "Height difference from the inlet to the outlet" annotation (
     Dialog(group = "Geometry"));
   parameter SI.Length L = 1000 "Length of the pipe" annotation (
     Dialog(group = "Geometry"));
-  parameter SI.Diameter D_i = 1.0 "Diameter of the inlet side" annotation (
-    Dialog(group = "Geometry"));
+  parameter SI.Diameter D_i = 1 "Diameter of the inlet side" annotation (
+    Dialog(group = "Geometry", enable=not use_A));
   parameter SI.Diameter D_o = D_i "Diameter of the outlet side" annotation (
-    Dialog(group = "Geometry"));
+    Dialog(group = "Geometry", enable=not use_A));
+  parameter Boolean use_A = false "Use areas instead of diameters" annotation (
+    Dialog(group = "Geometry"), choices(checkBox=true));
+  parameter SI.Area A_i = 0.785 "Area of the inlet side" annotation (
+    Dialog(group = "Geometry", enable=use_A));
+  parameter SI.Area A_o = A_i "Area of the outlet side" annotation (
+    Dialog(group = "Geometry", enable=use_A));
+
 
   // Steady state:
   parameter Boolean SteadyState=data.SteadyState "If true, starts in steady state" annotation (Dialog(group="Initialization"));
@@ -30,13 +37,15 @@ model Pipe "Model of a pipe"
   SI.VolumeFlowRate Vdot "Volume flow rate";
 
 protected
-    parameter SI.Diameter D_ = ( D_i + D_o) / 2 "Average diameter";
+    parameter SI.Diameter D_i_ = if use_A then sqrt(4 * A_i / C.pi) else D_i "Equivalent inlet diameter";
+    parameter SI.Diameter D_o_ = if use_A then sqrt(4 * A_o / C.pi) else D_o "Equivalent outlet diameter";
+    parameter SI.Diameter D_ = if use_A then sqrt(4 * A_ / C.pi) else (D_i_ + D_o_) / 2 "Average diameter";
 
-    parameter SI.Area A_ = D_ ^ 2 * C.pi / 4 "Average cross-sectional area";
-    parameter Real delta=2*(D_i-D_o)/(D_i+D_o) "Contraction factor";
+    parameter SI.Area A_ = if use_A then (A_i + A_o) / 2 else D_^2 * C.pi / 4 "Average cross-sectional area";
+    parameter Real delta=2*(D_i_-D_o_)/(D_i_+D_o_) "Contraction factor";
     parameter Real cf=1+2*delta^2 "Conical pipe function";
     parameter Real cos_theta = H / L "Slope ratio";
-    parameter Modelica.Units.NonSI.Angle_deg phi = Modelica.Units.Conversions.to_deg(Modelica.Math.atan((abs(D_i-D_o)/(2*L)))) "Cone half angle";
+    parameter Modelica.Units.NonSI.Angle_deg phi = Modelica.Units.Conversions.to_deg(Modelica.Math.atan(abs(D_i_-D_o_)/(2*L))) "Cone half angle";
 
 initial equation
   if useInitialFlow then
diff --git a/OpenHPL/Waterway/SurgeTank.mo b/OpenHPL/Waterway/SurgeTank.mo
index 5c3cc3b..b7cda65 100644
--- a/OpenHPL/Waterway/SurgeTank.mo
+++ b/OpenHPL/Waterway/SurgeTank.mo
@@ -3,7 +3,7 @@ model SurgeTank "Model of the surge tank/shaft"
   outer Data data "Using standard data set";
   extends OpenHPL.Icons.Surge(h_ds=h, H_ds=H, h_abs_ds=h_abs, show=data.showElevation);
   extends OpenHPL.Interfaces.TwoContacts;
-  extends Types.FrictionSpec( final D_h = D);
+  extends Types.FrictionSpec(final D_h = D_);
 
   parameter Types.SurgeTank SurgeTankType = OpenHPL.Types.SurgeTank.STSimple "Types of surge tank"
     annotation (Dialog(group = "Surge tank types"));
@@ -12,14 +12,22 @@ model SurgeTank "Model of the surge tank/shaft"
   parameter SI.Length L = H "Length of the surge shaft"
     annotation (Dialog(group = "Geometry"));
   parameter SI.Diameter D = 3 "Diameter of the surge shaft"
-    annotation (Dialog(group = "Geometry"));
+    annotation (Dialog(group = "Geometry", enable=not use_A));
+  parameter Boolean use_A = false "Use areas instead of diameters" annotation (
+    Dialog(group = "Geometry"), choices(checkBox=true));
+  parameter SI.Area A = 7.07 "Cross-sectional area of the surge shaft"
+    annotation (Dialog(group = "Geometry", enable=use_A));
   parameter SI.Position H_creek=H "Position of the creek intake above the surge tank base"
     annotation (Dialog(group = "Geometry"));
 
   parameter SI.Diameter D_so = D "If Sharp orifice type: Diameter of sharp orifice" annotation (
-    Dialog(group = "Geometry",enable=SurgeTankType == OpenHPL.Types.SurgeTank.STSharpOrifice));
+    Dialog(group = "Geometry", enable=not use_A and SurgeTankType == OpenHPL.Types.SurgeTank.STSharpOrifice));
+  parameter SI.Area A_so = 7.07 "Area of sharp orifice" annotation (
+    Dialog(group = "Geometry", enable=use_A and SurgeTankType == OpenHPL.Types.SurgeTank.STSharpOrifice));
   parameter SI.Diameter D_t = D "If Throttle value type: Diameter of throat" annotation (
-    Dialog(group = "Geometry",enable=SurgeTankType == OpenHPL.Types.SurgeTank.STThrottleValve));
+    Dialog(group = "Geometry", enable=not use_A and SurgeTankType == OpenHPL.Types.SurgeTank.STThrottleValve));
+  parameter SI.Area A_t = 7.07 "Area of throttle throat" annotation (
+    Dialog(group = "Geometry", enable=use_A and SurgeTankType == OpenHPL.Types.SurgeTank.STThrottleValve));
   parameter SI.Length L_t = L "If Throttle value type: Length of throat" annotation (
     Dialog(group = "Geometry",enable=SurgeTankType == OpenHPL.Types.SurgeTank.STThrottleValve));
 
@@ -39,8 +47,6 @@ model SurgeTank "Model of the surge tank/shaft"
   SI.Momentum M "Water momentum";
   SI.Force Mdot "Difference in influent and effulent momentum";
   SI.Force F "Total force acting in the surge tank";
-  SI.Area A = (C.pi*D ^ 2) / 4 "Cross sectional area of the surge tank";
-  SI.Area A_t = (C.pi*D_t ^ 2) / 4 "Cross sectional area of the throttle valve surge tank";
   SI.Length l = h / cos_theta "Length of water in the surge tank";
   Real cos_theta = H / L "Slope ratio";
   SI.Velocity v "Water velocity";
@@ -61,6 +67,9 @@ model SurgeTank "Model of the surge tank/shaft"
 
 protected
   SI.MassFlowRate mdot_creek "Creek mass flow rate (zero when creek connector is absent)";
+  parameter SI.Diameter D_ = if use_A then sqrt(4 * A / C.pi) else D;
+  parameter SI.Diameter D_so_ = if use_A then sqrt(4 * A_so / C.pi) else D_so;
+  parameter SI.Diameter D_t_ = if use_A then sqrt(4 * A_t / C.pi) else D_t;
 
 initial equation
   if SteadyState then
@@ -92,7 +101,7 @@ equation
     m = data.rho * A * l;
     M = m * v;
     p_t = data.p_a;
-    F_f = Functions.DarcyFriction.Friction(v, D, l, data.rho, data.mu, p_eps) + A * phiSO * 0.5 * data.rho * abs(v) * v;
+    F_f = Functions.DarcyFriction.Friction(v, D_, l, data.rho, data.mu, p_eps) + A * phiSO * 0.5 * data.rho * abs(v) * v;
     phiSO = 0;
     F_p = (p_b - p_t) * A;
   elseif SurgeTankType == OpenHPL.Types.SurgeTank.STAirCushion then
@@ -100,7 +109,7 @@ equation
     m = data.rho * A * l + m_a;
     M = m * v;
     p_t = p_ac * ((L - h_0 / cos_theta) / (L - l)) ^ data.gamma_air;
-    F_f = Functions.DarcyFriction.Friction(v, D, l, data.rho, data.mu, p_eps) + A * phiSO * 0.5 * data.rho * abs(v) * v;
+    F_f = Functions.DarcyFriction.Friction(v, D_, l, data.rho, data.mu, p_eps) + A * phiSO * 0.5 * data.rho * abs(v) * v;
     phiSO = 0;
     F_p = (p_b - p_t) * A;
   elseif SurgeTankType == OpenHPL.Types.SurgeTank.STSharpOrifice then
@@ -108,19 +117,19 @@ equation
     m = data.rho * A * l;
     M = m * v;
     p_t = data.p_a;
-    F_f = Functions.DarcyFriction.Friction(v, D, l, data.rho, data.mu, p_eps) + A * phiSO * 0.5 * data.rho * abs(v) * v;
+    F_f = Functions.DarcyFriction.Friction(v, D_, l, data.rho, data.mu, p_eps) + A * phiSO * 0.5 * data.rho * abs(v) * v;
     F_p = (p_b - p_t) * A;
     if v >= 0 then
-      phiSO = Functions.Fitting.FittingPhi(v, D, D_so, L, 90, data.rho, data.mu, p_eps, OpenHPL.Types.Fitting.SharpOrifice);
+      phiSO = Functions.Fitting.FittingPhi(v, D_, D_so_, L, 90, data.rho, data.mu, p_eps, OpenHPL.Types.Fitting.SharpOrifice);
     else
-      phiSO = Functions.Fitting.FittingPhi(v, D_so, D, L, 90, data.rho, data.mu, p_eps, OpenHPL.Types.Fitting.SharpOrifice);
+      phiSO = Functions.Fitting.FittingPhi(v, D_so_, D_, L, 90, data.rho, data.mu, p_eps, OpenHPL.Types.Fitting.SharpOrifice);
     end if;
   elseif SurgeTankType == OpenHPL.Types.SurgeTank.STThrottleValve then
     if l <= L_t then
       v = Vdot / A_t;
       m = data.rho * A_t * l;
       M = m * v;
-      F_f = Functions.DarcyFriction.Friction(v, D_t, l, data.rho, data.mu, p_eps) + A_t * phiSO * 0.5 * data.rho * abs(v) * v;
+      F_f = Functions.DarcyFriction.Friction(v, D_t_, l, data.rho, data.mu, p_eps) + A_t * phiSO * 0.5 * data.rho * abs(v) * v;
       phiSO = 0;
       F_p = (p_b - p_t) * A_t;
     else
@@ -128,11 +137,11 @@ equation
       m = data.rho * (A_t * L_t + A * (l - L_t));
       M = m * v;
       if v > 0 then
-        F_f = Functions.DarcyFriction.Friction(Vdot/A_t, D_t, L_t, data.rho, data.mu, p_eps) + Functions.DarcyFriction.Friction(Vdot/A, D, l - L_t, data.rho, data.mu, p_eps) + A_t * phiSO * 0.5 * data.rho * abs(Vdot/A_t) * Vdot/A_t;
-        phiSO = Functions.Fitting.FittingPhi(Vdot/A_t, D_t, D, L, 90, data.rho, data.mu, p_eps, OpenHPL.Types.Fitting.Square);
+        F_f = Functions.DarcyFriction.Friction(Vdot/A_t, D_t_, L_t, data.rho, data.mu, p_eps) + Functions.DarcyFriction.Friction(Vdot/A, D_, l - L_t, data.rho, data.mu, p_eps) + A_t * phiSO * 0.5 * data.rho * abs(Vdot/A_t) * Vdot/A_t;
+        phiSO = Functions.Fitting.FittingPhi(Vdot/A_t, D_t_, D_, L, 90, data.rho, data.mu, p_eps, OpenHPL.Types.Fitting.Square);
       elseif v < 0 then
-        F_f = Functions.DarcyFriction.Friction(Vdot/A_t, D_t, L_t, data.rho, data.mu, p_eps) + Functions.DarcyFriction.Friction(Vdot/A, D, l - L_t, data.rho, data.mu, p_eps) + A * phiSO * 0.5 * data.rho * abs(Vdot/A) * Vdot/A;
-        phiSO = Functions.Fitting.FittingPhi(Vdot/A, D, D_t, L, 90, data.rho, data.mu, p_eps, OpenHPL.Types.Fitting.Square);
+        F_f = Functions.DarcyFriction.Friction(Vdot/A_t, D_t_, L_t, data.rho, data.mu, p_eps) + Functions.DarcyFriction.Friction(Vdot/A, D_, l - L_t, data.rho, data.mu, p_eps) + A * phiSO * 0.5 * data.rho * abs(Vdot/A) * Vdot/A;
+        phiSO = Functions.Fitting.FittingPhi(Vdot/A, D_, D_t_, L, 90, data.rho, data.mu, p_eps, OpenHPL.Types.Fitting.Square);
       else
         F_f = 0;
         phiSO = 0;
@@ -213,7 +222,8 @@ $$ z_\\mathrm{creek} = z_\\mathrm{o} + H_\\mathrm{creek} $$</p>
 
 <h5>Parameters and Initialization</h5>
 
-<p>Required geometry parameters: length L, height H, diameter D,
+<p>Required geometry parameters: length L, height H, shaft diameter D or area A,
+and, for restricted configurations, orifice/throat diameters D<sub>so</sub>/D<sub>t</sub> or areas A<sub>so</sub>/A<sub>t</sub>,
 roughness p<sub>ε</sub>, and atmospheric pressure p<sup>atm</sup>. Initialize with flow rate V̇<sub>0</sub>
 and water height h<sub>0</sub>. Option for steady-state initialization is available.</p>
 </html>"));