The prophet Joel is drawing upon imagery and concepts from Ezekiel and Jeremiah, and the following paragraphs will help to tie the thoughts together.
First, the Day of Pentecost happened to be the same day when the Law of Moses was given on Sinai (Feast of Weeks). That is, The Book of Acts indicates that the New Covenant was given to Israel on the exact same day that the Old Covenant was given to Israel (Feast of Weeks = Pentecost).
As regards the New Covenant, there are four primary aspects to the New Covenant according to Hebrews 8:8-12, which is a quotation of Jeremiah 31:31-34...
- The forgiveness of sins
- The giving of God's Spirit
- The placement of God's law in the mind, and the writing of God's law on the heart
- The universal knowledge of the Lord
In Hebrews 10:11-18, we read that "at the present time" only the first three aspects of the New Covenant are in effect with one slight change. That is, instead of the placement of God's law in the mind and the writing of God's law on the heart, the law of God is instead written on the mind and is placed upon the heart. In other words, at the current time there is an emphasis and priority in the New Testament on the renewal of ones mind through the word of God (Rom 12:1-2 and Eph 4:22-24).
Second, the image of water raining down ("sprinkling") from heaven comes from Ezekiel. Ezekiel mentions the same four primary aspects of the New Covenant in Ezekiel 36:24-29 and Ezekiel 36:36. The water, according to Ezekiel, "cleanses from all filthiness." If this water is eternal life, which Jesus described (Jn 4:13-14), then the filthiness is not sin, but spiritual death. That is, it is blood that removes sins (Lev 17:11 or Heb 9:22), but only water was necessary for the removal of any uncleanliness associated with death (Lev 15; Lev 17; Lev 22; Num 19; etc.).
In other words, the outpouring in Acts 2:28-29 was the baptism of the Holy Spirit, which was the inauguration of the New Covenant in full force (like a downpour of rainwater from heaven). This baptism of the Holy Spirit however was dry, because the "water" was eternal life, which was poured out by, and through, the Holy Spirit. (Spiritual death was thus cleansed away.) The fire of tongues takes us yet back again to Sinai, when the words of God were written on tablets of stone on the Feast of Weeks. On Pentecost (which is the same day as the Feast of Weeks) that same fire was now putting the word of God not on tablets of stone, but on minds and hearts (2 Cor 3:3). The audible, dynamic articulation of the word of God resulted.
Therefore the prophet Joel had drawn upon imagery and concepts from Ezekiel and Jeremiah relating to the New Covenant. Thus on Pentecost Peter quoted him in Acts 2:28-29 regarding the "outpouring" of the Spirit in the New Covenant.