Shut is κλεῖσαι [kleisai] : shut, lock, bar.1 The same root is used where Jesus informs us many widows were in Israel in the days of Elijah, when the heaven was shut up three years and six months, and there was great famine throughout all the land [emphasis added] (Luke Luke 4:25b). Jesus described the ministry of Elijah the Tishbite who said to King Ahab, As the LORD God of Israel lives, before whom I stand, there shall not be dew nor rain these years, except at my word (1K. 1K. 17:1). Elijah, by the power of God, shut the atmospheric heaven from providing rain and dew.
so that no rain falls in the days of their prophecy
James refers to the similar event in Elijahs life to underscore the power of prayer in the life of believers. Elijah was a man with a nature like ours, and he prayed earnestly that it would not rain; and it did not rain on the land for three years and six months (Jas. Jas. 5:17). Both Jesus and James inform us that the heaven was shut against rain for a period of three years and six monthsa period of time matching the days of their (the two witnesses) prophecy : 1,260 days (Rev. Rev. 11:3+).
The third trumpet judgment resulted in the poisoning of one-third of the earths fresh water supply (Rev. Rev. 8:10-11+). Added to that, the three-and-one-half-year drought lasting throughout the 1,260 days of their preaching (Rev. Rev. 11:3+; cf. Luke Luke 4:25; Jas. Jas. 5:17) brought by the two witnesses will cause widespread devastation of crops and loss of human and animal life through thirst and starvation.2The lack of water and the sackcloth worn by the prophets allude to a time of fasting and mourning on the earth which is intended to produce repentance. See Who are the Witnesses? See Events of the 70th Week of Daniel.
power over waters to turn them into blood
The witnesses have power like that of Moses in Egypt (Ex. Ex. 4:9; Ex. 7:17-21; Ps. Ps. 78:44; Ps. 105:29). See Who are the Witnesses? See The Plagues of Egypt and the Tribulation. The results of this plague will emulate the effects of the second trumpet judgment and the second and third bowl judgments where the seas and springs of water become blood (Rev. Rev. 8:8+; Rev. 16:3-4+).
to strike the earth with all plagues
The two witnesses will be able to initiate numerous plague judgments upon the earth as they will. The full range of plagues is not described, but we can safely assume they are similar to the plagues with which Moses and Aaron tormented Egypt. See Who are the Witnesses? See The Plagues of Egypt and the Tribulation.
1 Frederick William Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature (Chicago, IL: University of Chicago Press, 2000), 434.
2 John MacArthur, Revelation 1-11 : The MacArthur New Testament Commentary (Chicago, IL: Moody Press, 1999), Rev. 11:6.