\documentclass[border=2mm]{standalone}

\usepackage[%
pdfauthor={$Author$},%
pdfkeywords={$Header$}%
]{hyperref}

\usepackage{tikz}
\usetikzlibrary{arrows,chains,shapes,fit,shadows,calc,decorations,matrix}

\definecolor{tulavendel}{RGB}{123,160,201} 
\definecolor{tufuchsia}{RGB}{161,0,88} 
\definecolor{tugreen}{RGB}{173,198,16}  

\begin{document}

\begin{tikzpicture}
% definitions
\tikzstyle{main}=[ellipse,fill=tugreen, draw=tugreen, drop shadow, thick, minimum width=20mm]

% nodes
\matrix[matrix of nodes,row sep=.7cm,column sep=-1.2cm, nodes={main}]{
\node (areacode) [] {Areacode}; & & & \\
& \node (theta) [] {Theta}; & & \node (wavedir) [] {Wavedir}; \\
\node (radius) [] {Radius}; & & \node (phi) [] {Phi}; & \\
& \node (erosionfac) [fill=tulavendel, draw=tulavendel] {Erosionfactor}; & & \\
};
  
% arrows
\tikzstyle{every path}=[-latex, thick]
\path (areacode) edge (radius)
      (areacode) edge (theta)
      (wavedir) edge (phi)
      (theta) edge (phi)
      (radius) edge (erosionfac)
      (phi) edge (erosionfac);

% rectangles
\tikzstyle{every node}=[rectangle, draw=black!100, rounded corners, inner sep=2mm]
%\node[fit= (radius) (erosionfac) (phi), inner sep=3mm,label=below:Learn from XBeach simulations] {};
%\node[fit= (areacode) (radius) (theta), label=above:Learn from geometry data] {};
\end{tikzpicture}

\end{document}