  Given an oriented graph G, the modular flow polynomial counts the number of
nowhere-zero Z_k-flows of G. We give a description of the modular flow
polynomial in terms of (open) Ehrhart polynomials of lattice polytopes. Using
Ehrhart-Macdonald reciprocity we give a combinatorial interpretation for the
values of the modular flow polynomial at negative arguments which answers a
question of Beck and Zaslavsky (2006). Our construction extends to Z_l-tensions
and we recover Stanley's reciprocity theorem for the chromatic polynomial.
Combining the combinatorial reciprocity statements for flows and tensions, we
give an enumerative interpretation for positive evaluations of the Tutte
polynomial of G.
