The easiest way of figuring out how to do this is to refer to a part of the game that does something similar. Russia, for example, gives extra yield to Tundra tiles, so I assume the same principles it uses to do so can be applied to a bonus for Desert tiles.
Investigation reveals that it does this through a series of Modifiers applied through the Trait (TRAIT_CIVILIZATION_MOTHER_RUSSIA, in this case). I'll give an example of two related parts as a reference for the general structure.
First, within the
<Modifiers> table:
Code:
<Row>
<ModifierId>TRAIT_INCREASED_TUNDRA_FAITH</ModifierId>
<ModifierType>MODIFIER_PLAYER_ADJUST_PLOT_YIELD</ModifierType>
<SubjectRequirementSetId>PLOT_HAS_TUNDRA_REQUIREMENTS</SubjectRequirementSetId>
</Row>
<Row>
<ModifierId>TRAIT_INCREASED_TUNDRA_HILLS_FAITH</ModifierId>
<ModifierType>MODIFIER_PLAYER_ADJUST_PLOT_YIELD</ModifierType>
<SubjectRequirementSetId>PLOT_HAS_TUNDRA_HILLS_REQUIREMENTS</SubjectRequirementSetId>
</Row>
These are the entries for how and when the modifier should act: in this case, it's an adjustment of plot yield on Tundra and Tundra Hills (I included both for a reason: it's important to note that the game considers flat and hill terrain to be two separate terrain entries; this is true of Desert and Desert Hills too, by the way, check Terrains.xml for more details).
Now for the the modifier's actual purpose, in the
<ModifierArguments> table:
Code:
<Row>
<ModifierId>TRAIT_INCREASED_TUNDRA_FAITH</ModifierId>
<Name>YieldType</Name>
<Value>YIELD_FAITH</Value>
</Row>
<Row>
<ModifierId>TRAIT_INCREASED_TUNDRA_FAITH</ModifierId>
<Name>Amount</Name>
<Value>1</Value>
</Row>
Two entries, the first defining what type of yield is to be modified and the second defining how much that yield is to be modified. This will give +1 Faith to Tundra tiles. I'm sure you can imagine what it'd look like for Tundra Hills (hint: the ModifierId is the key here).
And last but not least, actually applying these modifiers to the trait in the
<TraitModifiers> table:
Code:
<Row>
<TraitType>TRAIT_CIVILIZATION_MOTHER_RUSSIA</TraitType>
<ModifierId>TRAIT_INCREASED_TUNDRA_FAITH</ModifierId>
</Row>
<Row>
<TraitType>TRAIT_CIVILIZATION_MOTHER_RUSSIA</TraitType>
<ModifierId>TRAIT_INCREASED_TUNDRA_HILLS_FAITH</ModifierId>
</Row>
This was just a portion of Russia's trait, of course; it adds +1 Faith/+1 Production to Tundra tiles. Since Tundra Hills tiles are a specific type as well, it requires 4 different modifiers (two for each yield) to be applied to the trait.
If you wanted to apply this to Egypt, you'd apply it to the trait TRAIT_CIVILIZATION_ITERU and use Desert/Desert Hills instead of Tundra.