Preheat the oven to 350 degrees. Line two muffin tins with paper liners or spray with cooking spray. Set aside.
To make the streusel:
Combine the flour, brown sugar, cinnamon and salt in a medium bowl. Using a pastry blender, cut the butter into the flour mixture until the mixture looks like coarse crumbs. (If you prefer, you could also combine the dry ingredients and butter in a food processor and pulse to blend.) Stir in the pecans. Set aside.
To make the muffins:
Whisk together flour, baking soda, salt, cinnamon, and nutmeg in a medium bowl. Set aside.
In a large bowl, whisk together sugar, oil, water, pumpkin, eggs, and vanilla extract. Whisk until smooth. Slowly fold in the flour mixture. Mix until ingredients are combined. Add ¾ cup of the streusel to the mixture and stir until the streusel is just distributed throughout the batter.
Divide batter among the muffin cups and top each muffin with remaining streusel. Bake 18 to 20 minutes, or until a toothpick inserted in the center of muffin comes out clean. Allow to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
You can use walnuts instead of pecans, or omit the nuts, if desired.
Store these muffins in a sealed container for up to 3 days.