If you do hard labour in extreme weather conditions, and you fear exhaustion due to dehydration, and therefore, need to replenish your water intake,  you are allowed to forego fasting--only as long as you are working. You should make up for them later.  Allah says, "You should not kill yourselves, for God is Merciful to you." (Qur'an: 4: 29). We read elsewhere about the nature of the divine legislation,  "Strive hard in God's cause with all the striving that is due to Him: it is He who has elected you [to carry His message], and has laid no hardship on you in [anything that pertains to religion.." (Qur'an: 22:78).