f = Piecewise[{{0.35, week < 12}, {0.35 + (week - 12)*0.05,
12 <= week <= 22}, {0.85,
22 <= week < 33}, {0.85 + (33 - week)*0.1,
33 <= week <= 40}, {0.35, week > 39}}]

Plot3D[Piecewise[{{((moisture - 600)/320)*f, moisture < 920}, {0.35,
week < 12}, {0.35 + (week - 12)*0.05, 12 <= week <= 22}, {0.85,
22 <= week < 33}, {0.85 + (33 - week)*0.1,
33 <= week <= 40}, {0.35, week > 39}}], {week, 1, 52}, {moisture,