We know that the solitude of the figure described in this poem is of particular importance to the poet because the title itself describes her as a "solitary reaper." The title of a poem can be useful in giving us some guidance as to the poet's purpose in writing and as to their particular concerns.
In the first stanza of this poem, there are many other words which emphasize the fact that the reaper is by herself in the field as she pursues her daily business. In the opening line, she is described as "single," while the second line doubly confirms this with the word "solitary." For further emphasis, Wordsworth reinforces the image of the lonely girl working on her own with the phrases "by herself" and "alone." As she works, she is also singing, but we can assume from the nature of her song that she is not especially enjoying her solitude—her singing has a "melancholy strain," an evocative phrase which contributes to the mood of the poem. A single, solitary figure, performing repetitive work while filling the valley with the sound of her melancholy song, is a haunting image.