Ipnu Rivulet
Land — Desert
: Add 
.
, Pay 1 life: Add 
.
, , Sacrifice a Desert: Target player mills four cards. (They put the top four cards of their library into their graveyard.)
James Paick
Hour of Devastation (HOU)
• #180Rulings
If a Desert has an ability with a cost of "Sacrifice a Desert," you can sacrifice that Desert to pay the cost for its own ability.
(wotc • 2017-07-14)
Desert is a land subtype with no special meaning. It doesn't grant the land an intrinsic mana ability. Other cards may care about which lands are Deserts.
(wotc • 2017-04-18)
Rulings
If a Desert has an ability with a cost of "Sacrifice a Desert," you can sacrifice that Desert to pay the cost for its own ability.
(wotc • 2017-07-14)
Desert is a land subtype with no special meaning. It doesn't grant the land an intrinsic mana ability. Other cards may care about which lands are Deserts.
(wotc • 2017-04-18)