Complex sentence: Both Inner and Outer Terai have become Nepal's richest economic regions, with fertile farms and forests because of the area's generally flat terrain that is drained and nourished by several rivers.
Split into simpler sentences: Both Inner and Outer Tarai have become Nepal's richest economic regions, with fertile farms and forests. Tarai residents have better agricultural land than other Nepalese because of the area's generally flat terrain, which is drained and nourished by several rivers.