\begin{tikzpicture}[
    mysquare/.style={
      rectangle,
      draw=black,
      fill=white,
      fill opacity = 0.3,
      text opacity=1,
      inner sep=0pt,
      minimum size=40pt,
      font=\small,
      align=center},
      myarrow/.style={-Stealth},
      node distance=0.6cm and 1.0cm
    ]

    \node[mysquare] (c1) {};
    \node[mysquare,minimum size=20pt,draw=white,left= of c1] (e1) {$X_0$};
    \node[mysquare,below=of c1] (v1) {$V_1$};
    \node[mysquare,left=of v1] (v3) {$V_3$};
    \node[mysquare,right=of v1] (v2) {$V_2$};
    \node[mysquare,draw=white,below= of v1] (e2) {};

    \foreach \i/\j/\txt/\p/\r in {% start node/end node/text/position/rotation
      e1/c1.center//above/0,
      c1/v1/$ka$/right/90,
      v1/v3/$k_{13}$/above/0,
      v1/v2/$k_{12}$/above/0,
      v1/e2/$k$/right/90
    }
    \draw [myarrow] (\i) -- node[sloped,font=\small,\p,rotate=\r] {\txt} (\j);

  \draw [myarrow] (v3.340) -- node[font=\small,below] {$k_{31}$} (v1.200);
  \draw [myarrow] (v2.200) -- node[font=\small,below] {$k_{21}$} (v1.340);

\end{tikzpicture}
