E così?
`
\newcommand\pder[2]{\dfrac{\partial#1}{\partial#2}}
\begin{subequations}\label{MUDflow_COMPLETO_Ccostante_incognSURF_WATERdry}
\begin{align}
&\;\;\pder{H_1}{t}+\pder{q_{2,x}}{x}+\pder{q_{2,y}}{y}=0
\label{MUDflow_COMPLETO_Ccostante_incognSURF_WATERdry-a}\\[2ex]
&\left.\begin{aligned}
%
&\pder{H_2}{t}+\pder{q_{2,x}}{x}+\pder{q_{2,y}}{y} = – \dfrac{V_c – E_b}{C_m} + \dfrac{V_c – E_b}{1-p} \\
%
&\pder{q_{2,x}}{t}+
\pder{}{x}\left[ \dfrac{ q_{2,x}^2}{H_2-Z_b}+\dfrac{1}{2}g (H_2-Z_b)^2\right]+
\pder{}{y}\left( \dfrac{q_{2,x}q_{2,y}}{H_2-Z_b} \right) + \\
&\qquad g(H_2-Z_b)\dfrac{\partial{Z_b}}{\partial{x}} =-\dfrac{\tau_{b,x}}{\rho_{m}} \\
%
&\pder{q_{2,y}}{t}+\pder{}{x}\left( \dfrac{ q_{2,y}q_{2,x}}{H_2-Z_b} \right)+
\pder{}{y}\left[ \dfrac{ q_{2,y}^2}{H_2-Z_b}+\dfrac{1}{2}g (H_2-Z_b)^2\right]+ \\
& \qquad g(H_2-Z_b)\pder{Z_b}{y} =-\dfrac{\tau_{b,y}}{\rho_{m}} \\
%
&\pder{Z_b}{t} = \dfrac{V_c – E_b}{1-p}
\end{aligned}\right\}
\label{MUDflow_COMPLETO_Ccostante_incognSURF_WATERdry-b}
\end{align}
\end{subequations}
`