  This paper investigates how the far-IR water ice features can be used to
infer properties of disks around T Tauri stars and the water ice thermal
history. We explore the power of future observations with SOFIA/HIRMES and
SPICA's proposed far-IR instrument SAFARI. A series of detailed radiative
transfer disk models around a representative T Tauri star are used to
investigate how the far-IR water ice features at 45 and 63 micron change with
key disk properties: disk size, grain sizes, disk dust mass, dust settling, and
ice thickness. In addition, a series of models is devised to calculate the
water ice emission features from warmup, direct deposit and cooldown scenarios
of the water ice in disks. Photodesorption from icy grains in disk surfaces
weakens the mid-IR water ice features by factors 4-5. The far-IR water ice
emission features originate from small grains at the surface snow line in disks
at distance of 10-100 au. Unless this reservoir is missing in disks (e.g.
transitional disks with large cavities), the feature strength is not changing.
Grains larger than 10 micron do not contribute to the features. Grain settling
(using turbulent description) is affecting the strength of the ice features by
at most 15%. The strength of the ice feature scales with the disk dust mass and
water ice fraction on the grains, but saturates for dust masses larger than
1.e-4 Msun and for ice mantles that increase the dust mass by more than 50%.
The various thermal histories of water ice leave an imprint on the shape of the
features (crystalline/amorphous) as well as on the peak strength and position
of the 45 micron feature. SOFIA/HIRMES can only detect crystalline ice features
much stronger than simulated in our standard T Tauri disk model in deep
exposures (1 hr). SPICA/SAFARI can detect the typical ice features in our
standard T Tauri disk model in short exposures (10 min). (abbreviated)
